I'm glad you asked that question because I have wondered the same thing myself. I've logged just over 1500 miles on my bike and have not replaced anything except inner tubes and last week I replaced my pedals because I broke one of them. It was kinda a freaky thing how it broke.

I keep my bike cleaned up pretty decent and had it tuned up last year after the original couple hundred miles I put on it. I'd say right about now it's ready for another tune up just because. I am not experiencing any shifting or braking problems but it's just time for it.