Probably about 100,000 more. They run a ling time. The German 80s Mercedes in particular are TANKS, and anything Diesel is made to go the distance.
You must, of course, take good care of itm change all parts at regular intervals, and make sure your coolant does not corrode engine block passages. If you do this, and keep the engine clean. it should not fail for a long time. Corrosive stuff can kill it, though, its a tank but its still metal and steel.