I like the tire analogy, Racer. Rubber is not real rubber anymore....it's a synthetic mix. Impellers are not the quality they once were. I have many early OMC's with perfect impellers yet that are 50 years old. I replaced the auger drive belt on my 1970's vintage Toro snowblower after the original belt failed a couple years ago. Tried Kevlar, tried Toro OEM, they are all failing. Why? Nothing wrong with bearings, alignment, pulleys, gearbox, auger, or operating techniques. The replacement materials are simply not as good as in the 70's. Tires now are not made to last timewise. They are a composition that contains more synthetics. They are engineered for high miles, not like back in the "old days", when the average car or truck was used 10,000 miles per year. My 1976 Case backhoe loader has the original rear tires on it. I bought a new set of front tires 6 years ago and they are already full of cracks........modern materials used to make rubber compositions are inferior. Also, going back to impellers. Engineers have made water pumps higher pressure, in part by using tighter bends and shorter vanes on impellers. This further stresses the material as it rotates inside the housing. Vanes break down faster. Impeller designs are all different. The longevity will vary, depending on pump design, operating and storage conditions, as well as impeller material quality. As mentioned, pump housings must be considered because they can get pitted, depending on composition material, or distorted from wear or heat. So, it is very difficult to give you a certain timeframe for replacement.......yes, just like tires, Racer.