The rule of thumb on underwater sacrificial anodes is to check them at least once a year, but if you dry-store your boat, inspecting the anodes should be part of your wash-down routine. The anodes on outboards and stern drives can last a long time if the motor's lower end is out of the water when not in use, but a few days or even a few hours in a "hot" marina can silently dissolve the anodes and leave your engine under protected.
Outboards and stern drives are typically fitted with at least three anodes, often more, and every one is essential. When 50% of an anode has been lost to corrosion, replace it. If you do your boating in salt or brackish water, use zinc or aluminum anodes. If you boat only in freshwater, use magnesium anodes.