MB has never given such info. These measurements are seldom made in the modern shop. MB made provision for correcting this value through offset keys so there is never any reason why that cam timing can't be achieved no matter what the chain stretch.
I would guess that a chain with 3-5 deg of stretch should be replaced, BUT, there are other factors that change the cam timing including the decking of the block and either side of the head also cam or follower wear.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician