I replaced the chain and cassette on my commuter today. It’s been around 2 years and I hadn’t checked the chain wear in ages. The chain was at the outer limits and the cassette was starting to show wear as well. I long ago quit keeping track of mileage so I can only guess, but I probably had around 3,000-3,500 miles on the drivetrain.
Checking the wear on your chain is simple. Hold a ruler along the chain and measure from center-to-center between a pair of pins that are one foot apart. If the measurement is exactly 12″ your chain is not worn. Anything over 12 1/16″ and it’s time to replace the chain. If the measurement is at or beyond 12 1/8″, both your chain and cassette are certainly shot.
Some mechanics recommend replacing the cassette each time you replace the chain. Others suggest every other chain replacement. I’ve found that a worn cassette can dramatically accelerate wear on a new chain, and a new chain may skip on an old cassette, so if the cassette is showing any wear at all, I’ll replace it when I replace the chain.
I like doing this stuff at home, but if wrenching isn’t your thing, you can always check your chain wear at home, and if you’re at the point of needing a replacement, your local bike shop can do the work for you. Whatever you do, it’s best to not wait until the chain starts skipping on the rear cogs; once it’s progressed to that point, your worn chain is also damaging your expensive chainrings up front.