Either in terms of overall play time or just in one session.
The first time I played Oblivion, I was not aware I could lose track of 32 hours.
I've lived over an actual month in Skyrim, that includes eating, sleeping, and wanking
(That's .4% of my life)
[spoiler]#Gaming[/spoiler]
-
Longest in one sitting would most likely be Halo 3, on Bungie day in 2009, in the Bungie vs. the world playlist. Started at 2 a.m. and finished at 10 p.m. Technically it wasn't nonstop, I had a summer league baseball scrimmage from 7-8:30 in the morning, but it seems like a blur because of all the gaming. Three of my friends and I played the whole time together, honestly don't regret it at all, one of my favorite gaming memories.