Why it matters
With use, pins and rollers wear and the chain "stretches". Leave it too long and it ruins your cassette and chainrings, which cost far more.
Use a checker
A chain wear checker tells you in seconds. On 11- and 12-speed drivetrains, replace at 0.5%; on fewer speeds you can wait until 0.75%.
No checker?
Twelve full links of a new chain measure exactly 12 inches (30.48 cm). If they measure 12 1/16" or more, the chain is worn.
Make it last
Clean and lube it often, and always after wet or muddy rides. Use dry lube in dry season and wet lube when it rains.