International - the folks that made the engine - says it was designed to have a B-10 life of 250,000 miles and a B-50 life of 350,000 miles. That means 90 percent of them will still be going strong at 250,000 miles, and half of them will still be going strong at 350,000 miles - regardless of maintenance. So if they get good maintenance, then you can expect to have a good engine after 350,000 miles.
Long distance highway haulers that change the oil every 5,000 miles and properly maintain the air cleaaner, fuel filter, and coolant can expect over 500,000 miles. But normal short-trip pickup use should wear out the engine earlier. Mine gets excellent maintenance, and I expect it to last over 400,000 miles - if they don't throw dirt in my face before then. Only 163,000 miles so far, and it will be 8 years old next month.
That's the engine only. Tranny is a different story. Plan on overhauling the automagic tranny every 100,000 miles unless you pay the big bucks to have it bulletproofed by BTS or someone comparable. Or if you have a stick shifter, then consider the clutch to be a "wear item" like brakes or shocks that needs replacing periodically.