I've pulled a couple 16 hour sessions in WoW. That's probably my longest.
Honorable mentions:
FFVII - 11 hrs.
and various Christmas Days (Nintendo 64, Dreamcast, Gamecube, '87 (Legend of Zelda), '90 (Tetris, Punch-Out!!, and TMNTII... that was a day)
"Let justice be done though the heavens fall." - Jim Garrison
"Ask not your horse, if ye should ride into battle" - myself