Sadly, only 2-3 times... a year. My gaming group has been scattered to the winds, I'm afraid (three different states), so we only play when we get together... Oddly, this hasn't stopped me from developing approximately six campaigns' worth of material...
Currently, we play about twice per three months. Sessions last 4-6 hours. (I am a player in this game.)
When I'm DMing, I generally shoot for twice a month, with 6-8 hour sessions. Given that it's been over a year since I've run a game, however, and everyone's schedules have changed and grown more demanding, I don't know if that's a realistic expectation anymore. Poop.