On the rate of Leveling

There are two rates of leveling: first, how long do you have to play, to gain a level. Second, how long does it take in-game for your character to gain a level. 

In-game

How fast you gain experience in-game obviously depends on how your character spends his time. If he is engaged in uneventful overland trecks, weeks of downtime to craft items, or takes a break just relaxing and waiting, little experience will be gained, and it can take months or years. So the real question there becomes, how fast in-game can the character gain levels when engaged in the most strenuous activity, making most of every available resource or minute in an effort to gain exprience. 

"Experience" is licensed under CC-BY 4.0. 

In 5e it takes a little more than a month of in-game time to go from level one to level twenty. You'll spend approximately two days of in-game time on each character level, before moving on to the next one. Of course, with travel, downtime and so on, it may take longer, maybe two or three months of game time. But still, no years of heroics to become a world-dominating legend. 

In D&D 5e the standard experience rules that assign experience primarily for killing monsters, that means dangerous adventures with as many combat encounters as you can stomach, if you follow the DMG guidance on encounters per day and the XP per encounter, you get this table. 

Table 1: Experience and level achieved after number of days adventuring

Day    XP    Level

1 300         2
2 900         3
3 2,100 3
4 3,300 4
5 5,000 4
6 6,700 5
7 10,200 5
8 13,700 5
9 17,200 6
10 21,200 6
11 25,200 7
12 30,200 7
13 35,200 8
14 41,200 8
15 47,200 8
16 53,200 9
17 60,700 9
18 68,200 10
19 77,200 10
20 86,200 11
21 96,700 11
22 107,200 12
23 118,700 12
24 130,200 13
25 143,700 14
26 158,700 14
27 173,700 15
28 191,700 15
29 209,700 16
30 229,700 17
31 254,700 17
32 279,700 18
33 306,700 19
34 336,700 19
35 366,700 20

As a pure spell caster, you gain two new spells every spell level. When you gain a new spells up to character level 5 you get two slots to cast it, after that only one; followed by an extra slot on the next character level, up to character level 10. That means, when spending typically two days on each level,  in the lower levels you have 2, 2, 3, 3 = 10 shots to cast your new spells, before moving to the next spell level. In the mid levels you have 1, 1, 2, 2 = 6 shots. In the higher levels 1, 1, 1, 1 = 4 shots. Starting in the mid-levels, you get to cast each of your new spells maybe 3 times before moving on, in the high levels only twice. And this assumes that you do not find any additional spells adventuring on scrolls or from a captured spell books. If you find just a single new spell per level, then you will barely be able to cast each spell once before moving on mid-levels, and not even that in high levels.

This feels a bit fast for my taste. You do not really get to explore your newfound abilities, already you are on to the next shiny toy. If the progression was slower, maybe even by just a day or two per level, you could explore the spells more, get used to them. Of course, you still can use them later on, when you are on a higher level. But it feels a bit anticlimatic, if you finally get to polymorph someone on level nine. This would speak for slower experience. However, other classes gain less new features per level than spell casters, and for them the current rate may be just fine. 

In real-world time

How long should it take you to gain a level in play time? My answer is between six and eight evening game sessions. 

I think this is more important than how long it takes in-game, if you assume the main goal is to play a full campaign. The three important ingredients are and what level you need to reach, how long a campaign can run if you intend to finish it, and how often and long you get to play. 

In Gygax' original campaign old veterans had PCs with levels in the teens after about 10 years of play. [#8360]. 

That seems rough. With moving around for education and jobs, getting kids, changing life priorities and interests, any single game that requires you to contiuously commit yourself over 10 years is likely to be abandoned. Two years for a campaign looks doable. Keeping it up for four years is already much harder. 

A normal person for whom RPG is a pastime among others and not the center of their life will not play 7 days a week, as Gary did. We are playing about once per week for three to four hours of real time, and that is the same rate I have experienced in other groups, too. One evening a week is as good a rate as you can expect for a hobby, maybe dropping a few weeks each year to vacations. With busy jobs and kids and family, once a fortnight may be more realistic. 

So you are looking at a bit under 50 sessions a year, and you should be able to finish a campaign within two years or so to achieve closure. Campaigns do not have to go to level 20, in fact very few do or did, level 12-13 is entirely fine and may be preferrable. Even in Gary's campaign, nearly all players retired their characters around there. For this, you would need to gain about six levels a year, one level every two months or seven to eight sessions. 

Gary Gygax said I think that 52 sessions to reach 10th level is about right if the time per session is about four hours. [#5188], which is in the same ballpark.  This would come to about six sessions per level, that being action-packed sessions with theatre of the mind. With weekly play, it would take a year. He has other estimates that made it two years. 

We took nearly two years to get to level 10, but we only played biweekly in the first year, and we often play only three hours effectively, after kids are in bed starting around 9 pm, and ending around midnight with work the next day.  Our DM also is slow in adjucating, taking long time to look up adventure text, not feeling at ease with inventing on the spot to keep things moving, taking a lot of time to count squares in battle sitations. We have too many lengthy rules discussions, and virtual tabletop technical issues tend to slow things down further. So this seems to match the above rate given the extra impediments.

I expect it will take another two years to get to level twenty, if we can stick with it -- there is a bit of fatigue showing. 

Level 20

We never made it all the way to level twenty, and it would be a cool item for the bucket list to have played one character or campaign all the way through. All earlier campaigns and groups faltered after about 10 or so levels, the longest one went to level 14. 

Getting to level 20 is hard in older editions of D&D, because it took forever to get to the higher levels XP wise, but in all editions, because there are not a lot of good adventures for high level play, campaing arcs tend to end at level twelve or so, and the game also is not designed for high level play and less fun there.

Leveling In OD&D

In Gary's campaign, where the same character was played weekly, sometimes more often than that.

Good players could manage to gain low levels for their PC in a half-dozen or so adventures. Poor ones, those just goofing around couldn't manage that in a dozen adventures. [35] 

By the time AD&D was being played, all that had been ironed out, and the good players were still gaining a level for their PCs every couple of months until mid-level, say around 8th. [35]

The number of XPs given to rise a level was initially intuitive, later on based on the play of my campaign group. I think that 52 sessions to reach 10th level is about right if the time per session is about four hours. Longer sessions would reduce the number accordingly. #5188

A group playing once a week for three to four hours, playing well as a team, should see a 1st level PC that make about one level every three or four months on average. So that should get the typical party member to 9th level at the end of two or three years as you suggest. [35]

Its ambiguous what adventures relates to in the first sentence - game sessions? There were no adventure modules per se in the early days, only forays into the dungeon. This interpretation would mean six to twelve sessions per level, six with competent play. The third statements indicates gaining a level on average every six sessions (nine leves over 52 sessions), eight or more i.e. "a couple of months" towards the end, so maybe around four in the beginning. The fourth statement assumes much slower progression, one level per twelve or more slightly shorter sessions. 

The old veterans had PCs with levels in the teens after about 10 years of play.  [#8360]

A 15th level PC in AD&D requires years of gaming, and when arriving at thay level the character is generally retired. In new D&D arriving at that level takes a mere few months, and that PC is nothing compared to the half-dragon/half-vampire multi-prestige class one that the kid next door stomps around the campaign world with #3850

After name levels, this slowed down significantly, taking about two more years per level if you reached 15th after 10 years. This slowdown mirrors my own experience from second edition, and may also reflect that the high level PCs were played much less regularly. 

In-game time to level in OD&D

If play was intensive dungeon crawling, the 52 play sessions might take up only a few weeks of game time, with several adventure sessions being the continuation of a single day of delving. Also, when magically sent to another location time was generally different, and one reappeared in the original place with only a fraction of subjective time while away having passed in the home universe.  #5189

At  low levels if you gain ten levels in the span of a year of real-time sessions equaling "a few weeks" of dungeoneering game time, then your character would gain a level every two to three days of game time, same as in fifth edition. If it takes you two years and twice as many sessions to get to level 9, these 52 sessions and "few weeks" would only get you to leve five, and it would take you four or five days in game to level, half as fast as in fifth edition. 

Outdoor adventures might consume months of game time, of course. The latter posed a problem for players used to adventuring as a group when they were not with the others on an outdoor foray, so the regulars would often seek their fellows on such jouneys. To answer in general, the time span for 52 adventure sessions was generally anywhere from as many weeks to two years or longer. #5189

Also note that in OD&D, XP and leveling was defined much more by the treasure found, than by the monsters defeated. So if you made the mistake to hand out a huge treasure that was inapropriate, this could wreck havoc with accelerated progression. 

I am not particularly find of playing one game session and going up a level. That hardly qualifies as "eaned," to my way of thinking. However, if the campaign is set up for very high level play, such increase might be warranted. I did play in and enjoyed that sort of gaming with my French fellows, Francois Froideval being the DM. (My 12th level fighter was a mere peon, akin to a low-level PC, in that campaign.) [35]

I don't think you should ever allow a PC to gain more than one level from an adventure success. #1683