I completed Fable 2 in a day, but I suspect that's because I woke up at 8am, realised I didn't have to go anywhere and didn't put down my xbox until I beat that monstrosity of a game just so I could complain to my fanboy friends how horrible it was.
But I suppose that was particularly long and if I wasn't such a nerd would have last me a good week...I suppose the shortest game I remember playing (since I play in a state of comatose) would just so happen to be Metal Gear for the NES...took me about an hour and then BOOM ending, still couldn't resist a victory lap for having that on my list of completed games ^_^