MW2 (like just about everyone on release day)... I had nothing better to do that evening. 5 Hours.
Portal. Played it on a friends xbox at about 3am onwards after a party died down. Amazing.
Gears of War 2. Co-Op, was a laugh.
Halo 2. Co-op again.
Halo 3.
Hitman: Codename 47. Recently aquired and blitzed through in a day.
Star Wars Bounty Hunter *cough*. Had nothing better to do.
Most other games tend to have been over 2 days now that I think about it. I've almost played HL1 in one sitting... but I played through a short way for 20 mins or so, then went back to it the next day and completed it. I didn't mean to spend all day on it.