Each chain link is exactly one half of an inch long, so using a twelve-inch rule and measuring from the centre of one pin you can see when it has begun to "stretch". If 24 links are more than 12 and 1/16 inches long it needs replacing. Once they are 12 and 1/8 inches long you will be damaging your cassette and probably have to replace that, too.

Or you can buy the park tools chain measuring device and follow the instructions.