Use the exact engine manual, and act on whichever limit arrives first: running hours or calendar time. There is no safe universal interval, and an engine that has done very few hours is not exempt.
Two things age an engine that is barely used. Oil absorbs combustion by-products and moisture, and those attack bearing surfaces whether the engine runs or not. Coolant, impellers, belts, hoses and anodes degrade on a calendar, not on an hour meter. A yacht auxiliary that does thirty hours a year still needs its annual service.
The practical pattern for most recreational owners is an annual service built around oil and filters, a raw-water impeller inspection, a coolant check and a belt, hose and anode inspection, with longer-interval items — coolant replacement, valve clearances, heat exchanger service — scheduled by the manual's hour figures. Record hours, dates, part numbers and what you found each time; the record is what makes the next fault diagnosable.