Too many things to look at.  
From a Match rifle barrel standpoint here is my experience:
Around 4K rounds the 600yd scores will start to suffer - due to the throat going out.
that same barrel will still clean 300yds for another 2K rounds.
After that point the barrel is retired to reduced course matches for another 1K rounds or so, where it will still shoot 1-2" groups.

So depending on what kind of shooting you are doing, and at what ranges, I would say that a SS or chrome moly barrel will last the casual shooter at least 10K rds. For 95% of the AR owners, thats probbaly a life time.