For me, it all depends.
Most titles do have post-game content, which will keep you sticking for a while after you are done with the story-line. Me, though, I rely more on how entertaining those content happen to be. If they're not very important and are unable to give me any excitement, if at all, I'll most likely trade the game in and go for something new (every now and again, I'd go through the story one more time before doing so).
There are also titles out there that are known for their multiplayer mode and whatnot, such as Call of Duty and Super Smash Bros, and if it's one of those, then I will probably not trade the game in unless I no longer have the console to play it with.