Figure in 8 to 10 years. My 2000 FLD has right at 800k miles. Ohio CaT dynoed the snot out of it and said it behaved like a young engine, still getting up-to-spec results on horsepower at the wheel. A Class 8 engine like a Cat C10 or DD 60 series is designed to pull 80,000 lbs on a regular basis, between truck, trailer and cargo. In a straight truck application, you may be rolling as much as 40 or 42k on occasion. That class 8 won't work as hard. Just find out what the age requirements are with the carriers you are interested in.