Here's some good news then. I very much doubt that you'll be knocking out Assassin's Creed or Mass Effect in a single night. From what I understand, if you just plow straight through AC with no finesse, planning, and not even trying to explore anything, it'll still take around 20 hours gameplay. If you try to get the most out of it, then double that, if not more. Mass Effect, that should take a ton of time as well. The story itself takes long enough, but you never get everything in any single play through. The choices you make in game will determine which avenues open up to you. That doesn't even cover the fact that you have 3 different histories, 3 different personality types, 6 or 7 types of combat specialization, and the option to play as a man or a woman. Do the math on that, and you've got a ton of different ways to just start the game, and each one has an effect on how everything plays out. Thats a ton of gameplay. Mario might be a little on the short side, but you can always go back and play through as Luigi, so that might help lengthen it a bit.
Overall, just those 3 games should be enough to last quite a while. And there's still a ton of games out there. Gears of War just came to PC with expanded content. Theres the Orange Box with HL2, HL2ep1, HL2ep2, Team Fortress, and Portal that's available. Plenty of games out there. I'm sure you can waste a good bit of time.