Our MaxProp zinc lasts abot 3 years, of 6 month/ year sailing.
We also have two donut zincs on the shaft and replace one about every 3 months. Same whether in Arctic or tropics
Milvina is aluminium, with a zinc that is VERY slowly eroded on the hull
We have been very careful about keeping electrics separate from hull
Prop shaft is SS, and a Drivesaver isolates it from engine.