50+ provided there's a solid story to go with it. Something that is going to take, maybe a week to finish would be even better, and that long not because you'll take long breaks, but because there's that much content, alternatively, especially if it's an RPG, provide the content in such a way that it encompasses maximum re-playability. Or at least very good re-playability ^^'
For FPS'es...depends, obviously with an online affair like TF2 you will maybe play 5 or 6 matches a day, ranging from 20-30 minutes each, or longer/shorter depending. But for single player, again, provided a good storyline, 30 or 40 hours sounds pretty good to me.
MMO's are...different, and, if they hold your interest, go well over 50+ hours, many into the months, and years if you -really- like it...been playing WoW the past 2 years now I guess.