I replace my chains when the shifting gets noticeably worse. It's usually on one or two cogs. This is long before the chain starts skipping... that's really worn. On a Park chain checker, that's between .75 and 1.0 (on Shimano chains). I get 2000-2500 miles from a 10sp shimano chain. 9sp ones lasted a bit longer. Thats pretty often but my shifting is always good and the cassettes and chainrings last a long time.