I believe their good for 150k...200k..but like anything else if your budget allows no harm changing it...not sure what to check for to make a change necessary...some one on here will know...i'll wait and see
My local dealer hasn't canged any on 300TDs. I would think 200K is probably a good limit but I don't think Dr Nicks car has had one yet and thats on about 440k.
The amount of stretch can be checked by checking the cam and crank timing. More than five degrees means a stretched chain.