In my experience, none. No game can keep me enjoying it indefinitely. Once you've been through the story enough times, and done every side quest and piece of dialogue available, the game gets boring. Games like Mass Effect and Fallout 3 were big enough to hold onto my attention for four or five playthroughs, which was a very good number, but eventually I was just playing the entire game for one character romance I hadn't done, or one achievement that I wanted. Hell, I played ME2 three times just for the 'No man left behind' achievement, and all because in the second round, Mordin suffered a sudden case of stupidity and took a bullet while holding the line. I imagine Minecraft might have what it takes to keep me interested indefinitely, but thanks to fucking Paypal I can't get Minecraft, so I don't know.