How long does it take to complete Dead rising for the 360? cause i read somewhere it only takes 6 hours and i've been thinking about buying it for Wii. And i'm not paying $50 for a 6 hour  game

Forget the length. Have you looked at the wii version? Why would you buy that?

One run through of the main story is about 6 hours, at 3 days and 2 hours per day in-game. The Overtime mode adds 1-day/2 hours, which is still rather short, but a lot of the appeal of Dead Rising, at least on 360, was the added replay value due to the achievements.

Getting the Saint achievement your first playthrough is all but impossible. Zombie Genocider requires another playthrough all its own, plus there's the infinite mode where your only objective is find food and stay alive. If you do all there is to do you can easily sink ~50 hours into it, at least on 360.