Around 20 hours with Civilization (1 through 4), and I never planned to, with this game it just happens! You get up to get some breakfast, start the computer, and suddenly it's a couple of hours after midnight! Similar strategy games like the total war series are dangerous as well.
I have also spent long sessions with some zelda games, super mario world and super mario kart.
Nowdays I don't have much time for gaming, the longest was about 3 hrs on GTA IV.