In any level-based roleplaying game the people who run the game need to think about how fast they want the players to gain levels. There is clearly an optimum somewhere in the middle: Too fast and players get new abilities before they had time to try out the previous ones they got; too slow and players have the feeling their characters are stagnating. So while preparing my first D&D 4E campaign, I had to think about this.
My introductory adventure from level 0 to level 1 took just 2 play sessions. Or rather one-and-a-half sessions, with the second half of the second session spent "building" the level 1 characters. That was okay for the "tutorial", but I would consider this a bit too fast for the long run if my players leveled up every other session. On the other side we only play every 14 days, and even that is postponed sometimes when real life intervenes. So if I would say "lets gain a level every 6 sessions" that would mean only every 3 months, which is probably too slow.
The official 4E D&D rules help by giving good guidelines at least for the formalized parts of the game, e.g. combat. There are guidelines on how to design a "standard" combat encounter, and these are based on giving an xp "budget" and adding monsters to the encounter based on how much xp they give. The overall result is that a standard combat encounter gives 10% of the xp a player needs to level. Of course there are minor combats that give less xp and epic fights that give more. But as a fight that gives more xp also takes longer, I can do my xp math just on the standard fights. That works equally well for encounters like skill challenges or traps and hazards, which pretty much result in the same amount of xp per hour as a standard combat.
Where it gets a bit trickier is when it comes to roleplaying. On the one side you want players to roleplay, to interact with the NPCs, to discuss the situation among each other. But giving out xp directly for roleplaying is difficult, especially since naturally some players are more talkative than others. This is where quests come in in my campaign: If there is a murder mystery or similar situation, the players would get a quest which rewards them with xp for solving it. Thus the time spent roleplaying isn't perceived as "lost". And the leveling speed isn't slowing down just because the players spend a session with little or no combat.
The other use for quests is to give an added bonus to the "final boss fight" of the adventure. By handing out additional quest xp for having finished the adventure, the end stands out more. And as an added advantage there is a higher probability that with that added bonus the players level up, so you can handle the changes to the characters between adventures. To me that always made more sense than learning a new power in the middle of a dungeon during a rest period.
So, with the xp for roleplaying issue solved via quest xp, the different types of non-combat encounters give out as much experience as a combat encounter, that is about 10% of a level per encounter. A long adventure with 20 encounters gives 2 levels. And I think that my players will be able to do at least 2 encounters per play session, and at most 4 (Your mileage may vary, we are a bunch of middle-aged guys, not power gamers.) So my 20-encounter adventure should last around 7 play sessions, or about 3 months, for 2 levels gained. I think that is a good speed for us. I'd even say that the 10 encounters per level speed is good for D&D in general, although of course other groups will play more frequently, and/or get more encounters done per play session.
Is anyone playing the official D&D Encounters that WoTC organizes as weekly games in various game shops? I'd be interested to hear how fast you level in these. And of course if you have thoughts on leveling speed in D&D from your own campaign, feel free to comment.