There is no age or mileage interval for replacing the chain. The factory recommended method for checking chain stretch or elongation requires checking timing marks at a given valve lift. A good enough indicator is to set the cam sprocket timing mark against the corresponding mark on the #1 cam bearing then see how far off TDC the crank pulley is. Take the average of several checks.

I don't know that there's a hard cut-off for the eyeball method but my guess is you're good up to 5* ATDC. Anything more and you should roll in a new chain. IIRC there are no offset keys for your engine.

I'm sure someone will chime in with other cut-offs.

You should probably check IP timing if you keep the chain. Using the RIV A-B light (available for rent through the tool forum) is the easiest way to go but there are eyeball methods as well.

Sixto
93 300SD 3.0