Depending on their size and location on your boat, you would typically allow for at least half of the anode to corrode before replacing it. If you leave it to corrode further, it will no longer offer the same protection for metal parts. For some, replacing it could be as often as once a year. In other cases, it could take two or three years before replacement is necessary. By inspecting your boat regularly, you'll be able to accurately keep track of the rate at which corrosion occurs.