How to tell when to replace cassette?
Without experience, or obvious damage, a cassette gauge like the one from Rohloff, or mileage are your best options for deciding when to replace a cassette. For me, a good rule of thumb has been:


10 chains = 2 cassettes = 1 set chain rings


That is, I change my chain every 1200-1500km. I change the cassette on the 5th time I change the chain. And I change the chain ring set every other time I change the cassette.

This is an example of an obviously worn chainring.



This is an example of a worn cassette.