With the group I DM, we play about twice a month. We played two 4-hour sessions last month and they hit level 4 by the end of the second session. All of them were new players, but they were extremely focused and worked really well with each other. I think group composition and focus, as well as the DM's "style", can very much effect how much play time it takes to level up. But generally, with my groups that play well together, in our 4-6 hour sessions, they level up once, if not more, per session. Higher levels are the exception to this though, and it takes a few sessions to level up once they get past the level 10-12 range, at least from what I've experienced.