You need to measure your chain often. A 24 pin interval of new chain measures exactly 12 inches. By the time any 24 pin interval measures 12 1/16 inches the chain should be replaced. If a 24 pin interval has stretched to 12 1/8 inches or more the cassette or freewheel has probably worn excessively and will need to be replaced.
It's also possible that the chainring(s) have been worn out but they normally last much longer than cassette cogs or chains.

Al