Fresh impeller every spring. Don't leave an impeller in the motor over the winter. It will take a "set" and may fail sooner. If you do re-use an impeller, remove it for the winter and it will last longer.
It's kinda wierd. I've changed mine every year and some years it looks like it really needed it and some years it doesn't. The hours are almost identical every year give or take 8 hours. My view is: peace of mind is worth alot when it comes to my fun so why chance it? It's a $48.00 part. Cheap insurance imo.