My 1989 300CE has done 84,000 km. At what odometer reading should I replace the timing chain?

At about 150,000 miles, sixes don't break plastic guide rails, 150k is early but I have heard of issue,had a diesel break 1/2 link at 130k miles, sawed a hole thru valve cover,my 94 e320 has 230,000 I haven't changed it yet,