Anodes should be inspected every year as part of routine maintenance and replaced when the anode becomes 50% depleted.  Additional care should be exercised if the source of the fresh water is an on board, reverse osmosis water maker.  If the anode has never been changed (or more than two years has elapsed since it was last inspected), it is recommended to leave the anode in place as the head of the anode may have "welded" itself in place and there is a risk of severe damage to the fitting and/or water heater