My understanding is that zinc needs to be replaced for sure when it gets to about 1/2 of it's original size. I was told that if lets say it takes 6 months in your particular area and conditions for the zinc to lose 1/2 it does not mean that it will take another 6 months to lose the other half but that this will happen much sooner. Thus the need to replace. I basically leave the old zinc on and put a new zinc nearby. Next haulout I replace the older one and so on. This way I always have a back up zinc taking up any surges in electolysis, etc. Some people I know have more than 2 and/or put another one on the shaft insude the hull
, which also acts as a brake in case of shaft coming out of its slot.
Another thing to consider are your neighbors and their electrical
situation. I was told that the worst culprits are docked boats plugged in into shore power
, marinas' own stray power, etc. A friend when docking
for extended time in unfamiliar place drops a zinc "dolphin" off of his stern for added protection of the prop and the shaft.