Yesterday (and part of today technically ), my friends and I spent a little less than 12 hours playing Champions of Norrath for the PS2. For those that don't know, this is a fun, complex, and challenging mulitplayer game with both offline and online multiplayer capabilities. The game is divided into three different modes of play, which a geared towards characters of different levels (kind of like Phantasy Star Online's Normal, Hard, and V-Hard). Each is successively more difficult. Yesterday, we beat the entire second difficulty setting, and started on the final one. The game is clearly meant to be a multiplayer title, as the number/strength of enemies does not change significantly based on the number of players in your party.
Although I have had some pretty long sessions of Warcraft 3, Phantasy Star Online, Starcraft, Armored Core, Spawn: In the Demon's Hand, and a few others titles, this is by far the longest I have ever spent playing games (it would be longer, but in the past there would always be breaks for tv, physical activities, etc.).
Anyway, what is your longest marathon gaming session. Note: bathroom breaks and meals do not interrupt the time as long as you go straight back to playing games afterwards.