Water heater anodes usually last the life of the tank. Why? Because once the anode is consumed, the tank begins to corrode.
That said, the average homeowner or boatowner doesn't check or replace the anode in the water heater either because he/she doesn't know there is one or because the labor involved in replacing the anode is as much as replacing the entire water heater. In many installations, the water heater must be disconnected and moved or tilted to replace the anode.