These people who throw around big round mileage-per-chain numbers always make feel like I'm littering or something equally antisocial by not getting 12,000 miles per chain.
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Inside a Chainglider there is no visible wear on a 10,000km/6000m Rohloff sprocket, though it must be said the transmission is on it's third chain because a) I am a masher, historically tough on chains, and b) I had to retire one chain very little worn because I fitted a bigger chainring as my health improved (and also because I fitted a centre electric motor which requires a dished chainring to preserve the tread width). How totally outstanding this is -- for me -- can be judged from the 1000m/1600km I used to get on complete set of chain, sprocket and chainring before...
Generally speaking, I buy excellent quality KMC X8 or Y8 chains in bulk at sales by my "local" dealer, Chainreactioncycles, and replace them at about 0.5 percent wear, which in the normal course of events happens at about 4500km (so the bike above would be on it's third chain anyway). Do the better chains protect your sprocket and chainring longer? I have no idea but would hope so because, according to the KMC literature, there are constructional and material differences between their chains, more cutting edge chain tech being introduced the higher up the range. Maybe George's cheap chains are as good up to some wear point x, and if you never exceed that wear point, there is no difference. Note however that I don't go for the X1 at the very top of the range, street price about twice that of the X8, as I don't see that if offers much extra for double the money, especially inside a Chainglider where stainless steel is probably wasted.
My reasoning for running chains only to 0.5 percent wear is that the sprocket (mine is the original-model Rohloff sprocket) could be hard to get off, while a chain replacement is a doddle -- the biggest challenge is to get the Chainglider on exactly as you had it before, so that it will be utterly silent. Even a good quality chain is also the cheapest component in the Rohloff transmission. So I change the chain a bit early and preserve the sprocket and the chainring. See Martin's analysis above of the two possible approaches to chain wear.
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Both my Chaingliders are long since all paid up in extra mileage on chains alone, and again on sprockets (I use stainless or painted steel chainrings and they just don't wear inside a Chainglider). I've come to the conclusion that the Chainglider is the best bargain on a bike, especially if all your cycling life you had an adversarial relationship with chains. I'm at last comfortable with chains, as long as they stay inside my Chainglider.