Originally Posted by Matt Gaunt

This is good advice.

I also replace 11 speed chains at 0.5% (this is Shimano's recommendation FWIW) but 10 speed (and fewer) at 0.75%. I usually end up getting roughly 3000 miles out of a chain and I clean and re-lube regularly - there's no way I'd ever get close to 8000.

Other than effective elongation due to bushing wear, the other factor to consider is that the chain will become slacker (i.e. it will bend side-to-side more easily) over time, which can cause degradation in shifting performance. It's unlikely you'd get such pronounced side-to-side wear without corresponding effective elongation but it's worth keeping an eye on for the OP if he's getting that mega mileage out of his chains.