- I suspect chains that run inside fully enclosed chain cases probably last forever..
probably - lets try and make some mad comparisons
a chain that we used to use to drive the cam on a car would be fully enclosed and be very well lubed and it would possibly last about 100,000 miles, and I would guess a similar amount of energy would be transmitted through that as a cyclist puts through his, or indeed her chain
100,000 miles averaging 30 mph = 3333 hours, average rpm 2000?
3333 * 2000 * 60 / 2 = 200 million spins
a bike 58 gear inches pedalling 3/4 of the time = 812 spins per mile
200,000,000 / 812 = 250,000 miles
chains were often double chains so may be just 125,000 miles ? but then again, they're about half the length so back to quarter of a million miles
my apologies for the completely mad pointless wild guesstimate