I change mine yearly and keep the spares. I've yet to see one so bad that I trashed it. I do it more as a PM move than having the impeller go south at the worst possible time.
Some folks pull the plate off a couple times a year for a look-see but I don't think you can get a good look at the impeller condition that way, and by the time you get it out you might as well replace it.
I've yet to replace the cam though.
Capt. Douglas Abbott
USCG/MCA IV/C.I./M.I. 500-ton Oceans
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