I've seen 3.0's and 4.0's run for well over 350k and the only reason people trade them at that point is because everything else on the truck is worn out. There's another person on this forum with well over 400k on a 1988 Ranger, with the 4-cylinder. To answer the original question of how long will they last? I would say FOREVER!
I've had 2 of them and sold them way too soon