I was always told that when comparing hours to miles accumulated on a vehicle 1 hour was equal to 50 miles. So most gas engines are good for around 150,000 miles or 3000 hours. factor in the longevity of Diesels which should last 2 to 3 times as long as a gas engine and your looking at about 5000 to 6000 hours before you start seeing the evidence of engine wear. This is all hypothetial of course and depends on how well the tractor was maintined.