You can get years out of an impellor if you never start the engine dry.
... or about 30 seconds of dry running.
... so if you're one of these people who doesn't mind starting the engine with no muffs and hose if it's "only for a few seconds", the odds of having a problem are quite real. If you never start it dry, I have little doubt you could get 100 hours out of it without giving it the first thought.