Just 4 times.
CoD4: A few days after launch (ie, everyone had it by then). I used to go on some forums (CharlieOscarDelta, Gaming 4 Life) with some other guys, really nice people who I now miss greatly, especially since the forums have been deactivated and life has gotten in the way of their Xbox Live gaming. Anyway, I stayed up for hours with them playing CoD multiplayer and it was some of the most fun I've ever had online.
Modern Warfare 2: Played with my forum friend on Spec Ops for an entire night levelling up to Echo. One of the few good times I had with that game.
Halo: Reach: Ten hours blasting through the campaign with a friend on launch night. Good times only got better when that final cutscene rolled around and we were left speechless.
Assassin's Creed: Brotherhood: Multiplayer in that game is surprisingly addictive. I turned around... oh, look, it's 5:30!