you're absoultly correct, i did not answer your question.

i did check my cat manual and on standard lift chains BL40 they say measure a 20 link span and the measurement for new chain is 381mm and the maximum wear stretch should not exceed 392mm so it should not exceed 10mm stretch, if it does then replace the chains.
Of course the size of the chain does matter in measurements and they all vary to some degree but most of them i've checked run around 10 to 15mm maximum stretch before they are considered replaceable.

Hope that answers your question :o)