I just took a hy35 off at around 268,000 miles. Truck was stock for the first 250,000 miles. Injectors and some sort of box the last 20,000. Turbo feels as tight as a stock one. I'm thinking with good oil change and not to much abuse it could be longer than the motor. You actually are suppose to have no metal to metal, good clean oil ought to keep any wear out of it.
My hx 35-40 probably has 125,000 miles on it in three different trucks. I don't see any signs of wear in it. I am meticulous about oil changes so that may have something to do with it.