...for water pumps is TIME, not mileage based.
One of the failure modes is the plastic blades breaking down and bits of them floating around and blocking the coolant passages. That is bad.
But, like the others have posted, you do NOT want metal blades for the reasons they cited.
So, you must just own the idea that you must R&R the pump with a fresh OEM unit every FOUR years.
Do that, and you will be fine.
