I replace mine every other year. I have bravo 1 drives with external seawater pumps. I replaced the impellers a couple weeks ago and one was missing 2 out of 8 blades. The other one had a couple of tears.
If the pump does go out while your on plane, it will melt the interior of your rubber exhaust hoses really fast. They will melt before the engine temperature light comes on. Fortunately, they usually go out during start-up.