Thanks for the replies, all. I've played in all editions from OD&D to 3.5, so I know full well how fast d20 levelling is - I remember in 2E, we had a campaign where we played weekends - Friday and Saturday nights, though not regularly; it took the better part of a year to hit L11, and that was with a generous DM.
In my opinion, the quantity of experience needed to increase level is not as important as how generous the DM is who awards them. In our game, we follow these guidelines...
At the end of each game session your character will be awarded experience points based on its accomplishments, such as good role-playing, kill points, clever conquests, and successful avoidances (rather than just hack and slash). If experience points are only awarded for kills, then characters see each creature encountered as an experience point value rather than a role-playing opportunity. Many of each player’s best D&D memories comes from witty role-playing rather than from reading off a list of creatures killed.
As a guideline, lower level characters (1st to 5th) should play about two game sessions per level. Middle level characters (6th to 10th) should play about three game sessions per level. Higher level characters (11th to 15th) about four game sessions per level. Upper level characters (16th to 20th) about five game sessions per level. Epic level characters (21st plus) five or more game sessions per level.
That sounds like an idea... except how long are the sessions? I think WotC's market research found that people play for 4-6 hours at a time. I could easily make up some guidelines for levelling by hours played instead of encounters...
We do something similar, but we found that we want to advance quicker during the lower levels, and advance slower during the mid and higher levels. Character personality is created most at lower levels, where you need to be more creative to get things done. But there is much less you can do, so we want to move through them quicker. Once we gain power, then things are slowed down.
Yeah. I've alleviated the "can't do much at lower levels" thing a bit - more class abilities, spellcasters have unlimited cantrips, etc. In our vanilla 3.5 game, we've been quite creative in our gameplay; my wizard has grappled a couple times, and we've used tactical movement in fights.
The character development thing is part of the reason I want to slow advancement slightly at the lower levels - it gives the players more time to "get to know" their PCs and develop them. If you burn through L1-5, you have to concentrate more on class abilities and build than character background. I realize some people like mechanics over RP; I just happen to go the other way.
One other problem that we have resolved is unequal leveling. If someone misses a game or two, they start falling behind. We had one player who was a bit behind in XP and had to go away for a month. When he returned he was hesitant about rejoining the group over two levels behind the others in experience. When we told him that we changed to equal leveling for all characters, he changed his mind in a snap and was extremely enthusiastic about rejoining.
Yeah, that's kind of a tough thing to handle, and really depends on the group - you can't write a rule for that.
First off, how long are your sessions. If you're following the guidelines, you need 26 par CR creatures to hit level 3. After four sessions, you are saying you have defeated about 21 par CR creatures. That's a heck of a lot of combat to be blitzing through, but, maybe your sessions are six hours long.
Well, let's see... we play at the FLGS, so our time is limited to when they close, but we usually get 4-5 hours. Our sessions are a little light on RP and moderately heavy on combat; we seem to be averaging about 4-5 encounters per session, though I'm sure that will slow down as the combats get more complicated.
It would help if you put your advancement in levels/hour, rather than levels/session.
Hmm, levels/hour... Like I said, we're playing around 5 hours per session, and we've played 5(?) sessions, so about 25 hours. That would put our advancement at about every 10 hours (we're halfway to L3). It's kind of hard to recall, because we're supposed to play every other weekend, but things come up - we've had a couple sessions postponed, and a couple more cancelled outright due to RL taking charge.
I know we hit L2 midway through the third session - we were 60 XP shy and about to head into an old tower, and I asked the DM if we could level up before going in. We'd done some RP leading up to that point, so he gave us the 60 XP.

Right now a couple of us (the ones who have been there most) are sitting at 1900 XP.
Me, I game 3 hour sessions. We bump about every 4 sessions, which is perfect for my tastes.
So, every 12 hours. Sounds about right - 12-15 hours would be pretty good.
Secondly, bumping at such low levels has always been pretty quick, in any edition. Do you find that the speed of leveling changes over time?
Since our current campaign is only L2, I can't realy say. I don't recall enough of the last campaign (different group) to say, since that was a few years ago.
Thirdly, are you following the xp guidelines or are you dumping in ad hoc xp on top? In other words, are you giving the xp that they suggest?
I'm a player, not the DM, so I can't say how he's doing XP. I think he's dumping in some ad hoc stuff - RP, goals, etc., but as I said, we're also a little heavy on combat.
As an aside, that's another reason I want to increase the XP values: d20 seems to be based solely around "kill the monsters, take their gold"; it's an encounter-based system, where you get XP by overcoming challenges. There's no built-in buffer for RP, good ideas, and story awards. Back in 2E, our DM let us call out for extra awards that we thought deserved XP; he would approve (or not) and award us appropriately.
Now, as far as speed goes, the idea of going 12 sessions between levels is just boring to me. Sorry to be blunt, but, the idea that I'm going to spend three months at a given level means that, for me, an entire campaign would result in about 4-6 levels.
Amen.
This can be particularly bad if you have classes with multiple dead levels. Having just played a binder from levels 5-8, I can say that dead levels REALLY suck.
Yugh, yeah. I've alleviated that somewhat with the revised classes, so they're more fun to play.
As I said, 1 level/12-16 hours of gameplay seems to work for us.
That seems like a good goal.
Then you need to factor in the often molasses slow combat of levels 10+ in 3x, where it's quite possible that it will take an entire one of your six hour game sessions to play through one encounter.
Just wanted to chime in here - high-level combats are VERY slow. When we did an epic campaign (~L30), a single combat took upwards of two hours. That's why many groups don't go much beyond 15th-20th level.