Well, this is a very difficult poll.
It all depends on the game itself, after all.
For example, I wouldn't play Final Fantasy for more than 70+ hours. I know it is an awesome game but it doesn't have much except the gameplay and story itself to keep us playing.
On the other hand, Star Ocean Till The End of Time managed to get me playing it for more than 100+ hours, since this game has everything to keep me entertained and satisfied. It has rich gameplay, wonderful storyline, awesome music, and tons of quests to keep us playing and playing and playing again. Not to mention that it has the Universe Level, which is very challenging.
It's just an example. My point is, a game can never be too long if it is entertaining & satisfying enough.