I've played an ungodly amount of Super Street Fighter 4. And before it, for likely a much longer amount of time, 3rd Strike, although it doesn't show anymore whatsoever --goddamn it's amazing how badly you can suck at something you've put so much time into just from being away from it for a year or so. I might as well be playing with my feet.
I played Ultima Online for roughly two years straight, for several hours every day, typically in the dead of night when I should've been getting rested for school the next day. Will never touch another MMO again.
Singleplayer-wise, I tend to drop games as soon as I've had my fill, which comes on rather quickly, so the longest I've played a game was probably however long it takes to complete it. Probably something like Xenogears or Fallout 3 at ~90 hours.