I have an 8.1 496 Volvo Penta, 2005 with 310 accrued hours. What is the suggested frequency of change for replacing the water-intake impeller (boat only operated in Michigan's freshwater lakes)
I don't know of any recommended frequency. Most drive impellers are changed in response to engine cooling issues. You're lucky since VP impellers are very simple to replace vs. Merc. I'd change it if you don't know when it was last changed. Then it will last several years depending on your annual hrs.
That may be a little overkill. I'd pull the pump housing every couple years or so to make a visual. Keep a new impeller and gasket on hand. Missing or cracked vanes need attention.
I'll be transitioning from Merc Alpha II to VP next year. Most likely I won't check the impeller for five years.