I have a chain gauge. They are REALLY cheap. And I replace when it starts to show wear .... .75%. That happens at about 2,000 miles. If you clean your chain and drive train after every ride, you can probably get more miles. If you ride with a worn chain, it will eat up your cassette which costs more $$$$ than a chain. A cassette will last me quite a LONG time like well over 10K.