As mentioned, a diesel turbo should last indefinitely...and the best ways to keep the longevity are to keep the oil clean and to NOT shut the engine down immediately after a long hard run...let it idle for a couple of minutes for the turbo to cool down. Shutting down without letting the turbo cool will eventually kill the bearings, because oil left up there on engine shut-down can actually cook onto the metal...

Actually, I think that most g@$ turbos die because people don't know this.