ON Hunter's post referenced above

Timing Chains Don't Last Forever

"Timing chain stretch does not exist = it is wear on the pins and bushings = Wear Elongation Measurement."

I have been trying to get people to use ' elongation ' instead of 'stretch' for years.. not making any progress either...

"Chain elongation/wear is not corrected by an offset Woodruff key.."

Technically correct... but moot if the Offset Woodfuff Key brings the situation back to where the IP can be set properly relative to the cams position.... and their position relative to the crank.

That is the value of the ' slack take up mechanism' ... the oil pressure operated ratcheting tensioner on the non load side of the chain travel.

So to the original question.. Technically you replace it when the offset keys are not able to bring it back into compliance with specs..
But the posts by Hunter reflect normal thinking on this subject I believe... wear does happen on the sprockets... so optimal situation is to renew them at the same time as the new chain. A worn sprocket will cause a new chain to wear much faster... and most of its wear happens in the first 20-30 K miles... assuming normal lubrication rules being followed.

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html