If you have a Universal M-15 and it has been converted from raw water cooling to closed loop cooling, you probably have an impeller on the coolant loop. Mine came with identical Sherwood raw and coolant pumps. Raw has been swapped out for an Oberdorfer. However, retained the Sherwood for the coolant loop. The coolant pump is on the back side of the enginer. The Oberdorfer is too big and will not fit in the small space between the block and the bell housing. In fact, the Sherwood is so tight you cannot pull the cover and replace the impeller in place. You have to unbolt the entire pump, slide it out, replace the impeller and then reinstall the pump.
While I inspect the raw water pump impeller at least once a season, because of the tough access, I'll go three seasons with the coolant pump. And then I just replace it even if it looks good. A new impeller is cheap compared to the trouble of inspecting or replacing. Not sure what the normal life of the coolant impeller would be. On one hand, it is not exposed to salt, grit, etc like the raw water impeller. On the other hand, it is circulating hot fluid which could accellerate rubber aging. So far, when I pull the impeller every three years, it is pristine. Not going to push it to four.