I would say 3 years at the maximum before his numbers dramatically drop off. However, do not look for another 27 TD season this year. Because of that his numbers will decline a bit but not enough to take him out of the top 3 (LT, Priest, and Ahman). Over the next seasons you do have to watch out for injury though, which is a stupid comment but usefull to say because you must do that with everybody.
If winning isn't everything, why do they keep score? - Vince Lombardi