For games with levels like D&D, how frequently do you think PCs should advance, given typical play? HOw do you use the XP system to facilitate that, if at all (group XP, individual XP, level by GM fiat)?
I was just musing about how my 5e DMG suggests PCs should level up every 2-3 sessions, which seems very fast to me. I ran a Pathfinder AP (Curse of the Crimson Throne, converted) using Pathfinder Medium Track XP (party xp), the campaign went from level 2 to level 14, and for most of it the PCs were levelling up every 2 sessions on average, about 8 hours of play. This meant their power was doubling about every 4 sessions; I felt this rate really harmed the game, pushing the PCs up into the double digit levels where the system really breaks down. With my 5e tabletop 'Runelords of the Shattered Star' game (mashing up the Paizo APs Shattered Star & Rise of the Runelords) I'm aiming for a level-up rate about once per 5 sessions, or about 20 hours of play, about half the advertised rate. I think this should work well for long term play; I would like this campaign to run maybe 4 years, about 90 sessions or so, at 5 sessions/level that should take the PCs to 20th, though I'm fine if they cap out lower or we play awhile using the Epic Boon rules in the DMG. The default 5e system seems to support this sort of progression rate ok just by using mostly lower level monsters, using individual xp, and not being too generous on bonus XP; currently the PCs are level 5 after 15 sessions, with rapid progression to 3rd then slow thereafter.
Both those campaigns (Crimson Throne & Shattered Star) run/ran fortnightly. My recently resumed 4e D&D Loudwater campaign runs fortnightly, evening sessions so shorter, 3 hours or so, and the PCs (using party xp) have levelled up about every 4 sessions/12 hours for a long time - currently just hit level 27 after 96 sessions. 4e is so slow that we only get 1 fight per session and I have to give a good deal of bonus XP to hit that rate; it's still slower than the recommended 1 level per 10 hours of play, or 2.5 typical sessions - same recommendation as 5e.
I also have a couple weekly games:
The Ghinarian Hills is an online text-chat 5e sword & sorcery themed game with individual xp, I use standard monster xp and a fair bit of xp from other sources. After 74 sessions the highest level PC is 15th, so a bit over 5 sessions to level. The other PCs are in the 12-13 range. This rate works pretty well for online game, maybe a bit fast.
Finally there's my weekly tabletop Classic D&D Karameikos campaign. After around 13 months of weekly play the PCs are in the 8-11 level range, though the highest couple had been played previously in an earlier campaign and came in higher. A recently retired Thief PC who'd played from the start at 1st level hit 10th level. Typical advancement rate is about 1 level per 5 sessions, which is the recommended rate in the Rules Cyclopedia. I find this works well; I tend to get this through bonus XP rather than huge piles of treasure, though.
Overall I'm finding in my games that about 1 level per 5 sessions seems to work best, which fits with the norms in older games (eg Gygax recommended that a year of weekly 1e AD&D play should get a successful PC from 1st to 9th), a bit quicker with 4e. But this is about half the default rate recommended by the GM guidance in 3e/PF, 4e and 5e, which all seem to recommend 2-3 sessions to level and 20 sessions in a year of weekly play. What do you find? What works best for you?