Alternating Zincs
Apparently a common practice here is SF Bay is to have two shaft zincs.
The diver that scrubs the bottom comes around on a contract every three months. At that time the diver switches out ONE of the zincs. Thus you always have (sorta) one that is approaching 6th months old, and the other is nearing three months old. The older one of course gets switched out. Been doing this for over 15 years on our 32-200 and it works really great.
Frankly, I think it all depends on the quality of your docking situation...marina (good or bad) mooring balls, no shore power...fresh or salt, etc.