I am not sure, probably 6-8 playing Grand turismo when I was young, I have done at lest that much playing C&C Generals, I was hooked for about 13 months.
The most time i've ever spent is probably around 8-10 hours playing KotOR, that game is just too hard to stop playing once you get into it!
The second longest is probably around 6 hours playing Halo with a few friends, we played that almost all night till we finally fell asleep.
13 Hours on Diablo2 & Expansio 15 Hours on Runescape (Regretably) 10 Hours on BF2 That's my longest so far. I had copious amounts of Caffene, tobbaco, and gamers munchies.
18 hrs on Enemy Territory Fortress when it first came out earlier this year. 10 hrs on Wofenstein: Enemy Territory when I got it waiting for the Fortress mod to come out.