The chain should measure 12" from pin to pin. At 12 1/8" you need to thiunk about it, at 12 1/2 you need to replace it. As long as you keep the chain changed the cassette will hold up for a good while. I've got 4,400 miles on my Enduro and have only put two cassettes on it and changed the mddle and big rings once. Look for the shark fin look on the teeth of the rings and cassette to judge the wear. The casstte wears to the chain and if you don't change the chain when you should and put on a new one the chain will not seat properly and make a grinding noise and jump around on the ring. Park tools makes a neat little tool that measures the chain wear also.