All artists build to a peak and most go off it before they stop performing or composing. Kanye West is pretty well the king of rap and the thing about albums is they last well after a performer has ended his career. But I think he has time on his side and if you disregard media hype, his own self image seems good.
The thing is, at that intense level let alone travelling to different countries, there can be burn out. I'd guess he has a good two years yet before the quality of his work goes off the boil.