On my 8 speed chain in bad weather/winter, I get 1800 miles until the chain is worn until it "should be replaced."

However, I found that from a $ standpoint, replacing it at that point makes no sense. If I replace the chain as soon as the chain gauge says to, I find that I still have a worn out cassette at about 9000 miles and have to replace that as well.

If I just leave the worn chain on, it still works just fine, and I can go to 9000 miles and even farther, then replace the chain and cassette at once then.

That way every 9000+ miles I go through one chain and one cassette rather than 4 chains and one cassette. Performance is the same either way.

I only this year got a nice weather bike, I don't really know but it has 2400 miles on it and the chain is not even to the 75% worn mark yet. Good thing too since chains and cassettes cost 3x as much for this bike, for no good reason that I know - other than "that's what people will pay for them." I'm unconvinced that slightly narrower chains cost a lot more to build.