That doesn't seem that out of line to me. I would consider 100 hours per impeller pretty good but would still like to know why they are going bad. Mine gets changed every season which is pretty much 50 hours.
A lot is also going to depend on how/where you use your boat. Brackish or saltwater will make short work of a impeller as well as dry starts. You're going to provide a lot more information to have anyone come up with any ideas.