You could change it on some schedule, maybe yearly with your extensive use. But changing the impeller on that engine is a PITA, so here is a quick way to see if it and your cooling system is performing ok:
Warm the engine up and take it out at cruising speed. Then after the temp stabilizes run it at wot and watch the temperature gauge for five minutes or so. It should not go up more than about ten degrees and stabilize at 200-210.
This test does more than check impeller condition, it checks the complete cooling system. If the impeller is at the end of its life and isn't putting out enough flow then the engine temperature will rise too far, but other causes like a fouled main heat exchanger or injection elbow will do the same thing.
Also while you are running at wot, check to make sure that your engine reaches rated rpms or a bit more. If not maybe you have prop or bottom fouling that needs to be taken care of.
David