Engine makers use an engine-industry standard to design, test and estimate a product's life span. The result is the B rating, where B means the engine will produce power intermittently, with full power demanded no more than 80 percent of the time. (There are also A, C, D and E ratings based on different criteria.)
To the B is added a number indicating a percentage of a model's total population. For example, Paccar says its 12.9-liter MX diesel has a B10 rating of 1 million miles (the organized trucking industry's long-stated goal for over-the-road service). That means only 10 percent will not make it that far without rebuilding, or, to be more optimistic, 90 percent will. That's pretty long-lived, though the North American MX, while based on a proven product from Europe, is rather new and must prove its longevity in actual service.
Detroit Diesel says its DD13's B50 rating is 1 million miles, which means half of those engines should go that distance without rebuilding and half won't. The B50 for the DD15 and DD16 is 1.2 million miles. Detroit has many years of experience in such matters, so the estimates should be reliable. Other makers quote similar numbers. And careful maintenance of any engine should extend its life.
How long-lived must an engine be? That's up to each customer, the builders say. Someone operating on a three-year trade cycle can expect a tractor to run 300,000 to 400,00 miles in that time, so can choose a lesser-rated engine with a higher B number. A longer trade cycle, maybe of five or 10 years and a million miles or more, will require a rating with a smaller B number.
For the B rating of the engine you're considering, ask the sales guy who wants you to buy it. The number should be available in the builder's literature.