The group I play in now meets on a two-weeks-on, one-week-off schedule, and the individual sessions are generally 4-6 hours.
I fondly remember the weekly gaming sessions when I was in high school that would often become 8-12+ hour sessions... but there's no way I could do that anymore. Nor do I have the desire or attention span for that -- I'd rather get home a little earlier and spend time with my wife.
From noon to 6:30 pm every other week. I would say that translates into 5.5 hours per session if you subtract 30 minutes at the beginning, and 30 in the middle for dealing with ordering food.
My Tuesday night Basic D&D game goes for 2 hours, 7:00 to 9:00. That game runs every Tuesday throughout the year.
My main campaign, D&D 3.0, starts up every October and goes until May or early June. During that time we meet every other Sunday from Noon till 6:00pm.