My 1977 B7100 had over 7500 hours on the meter and it had stopped working. Hard to say how many hours it actually had on it. They are REALLY tough little machines.
5 digits IIRC on both of my machines. Change the oil and don't let them overheat and you should get 10,000 hours out of one before tear down.
I did have to replace a water pump and an alternator on my Heston 90 100 DT after about 2500 hours. My L3710 has about 2200 hours on it and runs like the day It was dropped off at the farm. Delvac oil and watch the temp.