I'm nowhere near as beastly as some of you guys answered on the pole, 12+ hours on 1 game holy crap, but I usually get in a 3 to 4 hour sit once a week minimum.
Longest I ever sat and played 1 game, that I can remember, is Costume Quest. Snowed in, so I bought Costume Quest and beat it in 1 sitting. For me it was about a 7 hour game with all trophies gotten.
I'm sure I've done longer, not 12+ though, with Gears when it first came out(and it was my first multiplayer game) but I can't remember how long it was.
Most recently I went from 3 am to 9 am on Fallout 3, because I died 3 hours in without saving(had auto-saves off) so I played another 3 hours to make up for lost time.