I have had chains that had to be replaced at relatively low mirage and others that have lasted season after season. The last time I had one replaced at my LBS ( i usually do it myself but did not have time) the technician recommended changing the cassette at the same time, theory being no use putting a new part, the chain, on top of one that is at least partially worn. I trust this guy and the shop so I had it done. Seemed to make sense.