Only 37 votes, but it is interesting to note that 22 of them fall into the 3-5 category. I suppose this is what is advocated in the RAW.
I'm wondering if there is a direct correlation between speed of leveling up and the "dungeoncrawling-to-roleplaying spectrum." What I mean by this, is to what degree the focus of the game is on combat and dungeoncrawling vs. to what degree it is focused on roleplaying. I'm guessing that the more towards the former, the more frequent the leveling up - as a general rule, at least.
My game, for instance, is very much focused on combat and dungeoncrawling - more so than I prefer, but somewhat of a necessity given my busy life and lack of prep time. If I ran the campaign that I would really like to run, leveling up would probably occur less frequently. But because my current campaign is pretty much jumping from one adventure to another with only a small amount of roleplaying and character development, then leveling up more frequently makes sense.