Anode: how often should they be changed

[jack daniels marine]

I have a 2 stroke 40 hp DT40 suzuki outboard. There's a trim tab anode which looks pretty much brand new and a second anode on the transom clamp bracket which looks allmost new.

The anodes have probably been there a few years. Boat trips are only in saltwater, however the boat is allways parked up in a freshwater river when not in use with the outboard raised.

Do the anodes loose their protective ability with age even though they look reasonably new?
should I replace them each year regardless of how they look? The 1997 motor shows no visual signs of corrosion.