There are two schools of thought when it comes to replacing your chain. First of we should be following proper chain maintenance habits to get the most life out of our chain. One plan is to replace the chain early and often and the flip side is to wait until the chain, cassette, and chainrings are completely worn out and replace them all together.
As a chain wears out, first the pins and rollers wear down, this is referred to as “chain stretch” After riding with a worn out chain, the teeth on the chainring and cassette will begin to slowly wear down with it. This wear happens very gradually in such small amounts that it is difficult to see at first. There are some inexpensive tools that measure the wear on the chain that can give us valuable insight into the life left in the cassette. Once the teeth on the chainring or cassette have worn down enough they could allow the chain to slip over-top the cassette cog or fall off of the chainring.
Ideally the chain is replaced before this wear begins to set in on the cassette and chainrings. This means before reaching 75% stretch for most chains, and before 60-70% stretch on 11 and 12 speed chains. Once these thresholds have been reached, the only option to keep everything compatible with each other is to replace the chain, cassette, and chainrings together. Installing a new chain on the worn cassette and chainrings will cause skipping and grinding. We thoroughly test ride all repairs to ensure compatibility whenever installing new chains. Otherwise if one is not ready for a new drivetrain, you can continue to ride the current components until they reach 100% or shifting issues begin to creep in. At that point you know that every last possible mile was squeezed out of the components and a whole new drivetrain will make your bike feel like new again!