It varies for me.
Back in the day, on weekends of course (thank you jr. high and high school), I would play from the moment I woke up until I got tired enough to sleep.
Nowadays it all depends on the game that I'm playing.
I would say on average about 4-5 hours a session when I get really into it.
Although there is many a time when I can only squeeze in about an hour, give or take, depending on if I'm busy or if I just get ADD and feel the need to do something else.
I would probably say the longest I've spent on a game is about 16-17 hours.
Haha.. which brings me back to one thing.
One day my best friend and I spent, like, 3-4 hours playing Super Smash Bros. on the N64 on 100 stock with Bomboms and mines only, with the television hooked up to surround sound, and the volume turned up massively.
It's kind of lame that we did it that long, but we could go on for hours doing stupid stuff like that all the time. :D