I have a diver clean the hull and replace zincs as needed about every 3 months. Haul and recoat bottom at about 3 years. At that point, there are a few small (qurter/half-dollar size) and localized barnicle accumulations on struts and shafts. The bottom would probably go 4 years on the basis of coating alone, but I check other aspects as well.
The boat slip is in brackish, not salt, water. Spends 6-8 weeks per year in saltwater, but transits back each trip in at least 30 miles of brackish. I don't know if this makes a difference versus full time saltwater exposure.