I don't think it'll last, but I've been wrong before.

It's really fun, but I do not think the multiplayer team based games will last too long at the top. The free mode might have a chance at some staying power along with the single player. Because of the scope of maps it causes the team games to get old fast if you do not have a cohesive team communicating. You know it will stay up top for a while though because of the single player, which you dont have to be playing in an actual multiplayer game to be counted as playing on live...look at bioshock at it's peak.

I agree with your 4-8 week estimate. Although about the time it starts dying off there may be a spike up due to purchasers that did not get the game on release for whatever reason.
I almost have to make myself play the game already. Don't get me wrong it's REALLY fun, but I'd rather be playing something else with more of my friends.

Perhaps the extreme popularity of the Grand Theft Auto franchise has made everyone and their mothers buy and play the game since it's GTA. However, because the gameplay is not as good as people expect because people tend to over rate things, the game will get stale to some people quicker than others (see how COD4 prevailed over Halo 3 on the Live charts).

GTA is one of those games where you can just pick up the controller for a few minutes and have fun doing absolutely nothing. After you beat the story mode, you did not have many options to do afterwards minus the random side missions and total destruction. I could see myself playing GTA online for a long time for a few hours a week, once I get highspeed that is. I think it will stay in the top 5 games for a loooong time.

I never really got into GTA myself, but I have watched people play it and it looks awesome. I can only imagine what kinds of chaos can be unleashed with a mob of players as opposed to just one.