If we're talking real time, not in game time... Well... There's probably something longer, but recent memory I would say Final Fantasy 13... because I got it on launch day and didn't finish it until after I bought FF13-2 (Which I bought a few months after it was released)... So... approximately 2 years.
In game time, probably Tales of Graces F, game is fucking long as hell... took me 80 hours to finish, and I wasn't really dawdling with side quests or mini games or anything... I pretty much went more or less straight through the main story.
Not to say it was bad... I actually really enjoyed it... is the only game I've played start to finish that long without touching another game.