How often do/should players level up?

I see all the ways to gain XP on page 27, but for game balance I'm curious how others have let leveling up happen.

First, I plan to follow the side bar tip about limiting XP for failure, otherwise it seems like players could get multiple levels in a single session/episode. Though maybe I'm wrong in my assumptions and the players don't fail as much as I think they will?

More importantly though, it seems like a good idea to try to keep all players on the same level. So I was thinking of using a 'milestone' style of leveling instead. But then in the published adventures (like River City), about how often should players level up to be close to max by the end, but not too much before?

I think it depends on your players. I tend toward letting them level every game session if they're younger. Tweens and teenagers have more fun if they have to work at it for a session or two, in my experience.