I changed mine at 43,000 miles after reading all these horror stories - there was only very slight wear of the black layer and no indentation whatsoever
The inside obviously had a mark from the pump, but overall it was nice and shiny!
Like others, I changed it anyway as I had bought a new one, but I'm pretty sure it would have been fine for another 40,000 miles!
20k miles were done at Revo Stage1 and 10k at ST2+....I will try and post pics later.