John Stevenson wrote:
A good quality steel ruler, marked in old money, is your friend.
Chain links are exactly half an inch from pin centre to pin centre, so 24 links should be 12 inches long.
As a chain wears, this distance increases because the pivoting surfaces inside the chain get sloppy.
If 24 links are are 12 1/16in long, then it's time to think about replacing the chain and if they've got to 12 1/8in, you will probably have to replace the sprockets and chainrings too, as the lengthened chain will have worn them to the point where a new chain will slip.