As it wears, it appears to stretch, on a new chain, the links are all exactly 1/2" long, so if you take a ruler which is exactly 12" long, and align the end of one link with the 0, the end of another link will fall exactly at 12". As the chain wears, the place where the end of the link falls moves further and further away from the 12" mark. Usual rule of thumb is to replace the chain when the measurement across 12" stretches by 1/16" of it goes much further then it will result in wear to the cassette and you will have to replace that too. The cleaner and better lubed you keep the chain, the longer it will last. There are special tools to make the chain wear easier to measure if you do it a lot.