I've always heard 1/8" stretch is acceptable above that you should replace or plan on replacing the cassette sooner. As I result I normally get around 13-14,000 miles on a cassette and around 2500 miles on a chain.

Topic open for debate:

I typically ride bigger gearing than most people. Since I recent pretty much switched all the way over to single speed riding, even down to riding a single speed chain now, I've been riding 53x15 all the time. I keep wandering does the gear choice also determine the chain stretch over time...aka the bigger the gear the more power you have to apply to the pedal/chain and the sooner you end up having to replace the chain as a result. Does that idea make any sense or am I crazy for thinking that thought?