As a player, I am most comfortable levelling up every 4th session or so - I don't feel I get a good grasp on the abilities and tactics available to the character in less time, and it feels very odd to level up when I still feel lost in the options of the previous level. The campaign I play in tends to level every 2-3 sessions, which is somewhat frustrating to me. So far, I have dealt with this by playing the characters as increasingly out of their depth til they die, so I can bring in new folks who I feel more comfortable running.