100K is a good rule of thumb. Cheap preventative maintenance.
Just before leaving on vacation I stopped by my local Indie Franz, he just got a 420 in with 130K and a broken chain rail. He had been advising the owner to change the chain for the past 20K miles, owner didn't want to spend the $400-500 to do they job, and now looking at $2500-3000. The upper left chain rail broke, jumped time and took out all the rockers on the left head.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.