I'm not particularly proud of beating games fast, but Brothers, albeit an indie game with an expected shorter completion time, was done in about two hours give or take. I sat down and just beat on that game.
I always think, personally, that games should have 20+ hours of base content including any sidequest and collectibles. Although, of course this needs to vary from game to game considering I wouldn't want to play twenty hours of Call of Duty for a story that's irrelevant. But, to each their own - right?