nrg-b - to be honest I have no idea!
I'm still wearing the first pair of RC150s I bought a while back (although I do tend to alternate with a newer pair just because the current version tends to rub in different places to the old one - the old blue RC150s have a stiffer, almost plasticy, nylon mesh upper and they fit a bit differently to the current red and white version).
I would guess I must have run over a thousand miles in my oldest pair (and very possibly double that), and they feel just as good, if not better, than when they were new.
Isn't running properly(!) great - not only can you get away with ultra-light and cheap shoes, they also last for ages!