Zincs
The greater the salt content of the water the faster your zincs will corrode. Ask other boaters from your area to get the best idea on how long theirs last, then work from there. I stay in the water all year and can get close to two years from mine but understand I'm in the northern part of Chesapeake Bay which has a low salt content. It would never last that long if I were in the ocean. Just err on the side of safety. Better to replace too often than not often enough. The cost of a new zinc plus labor to replace is much cheaper than a new prop and/or shaft and associated labor. As far as bottom cleaning is concerned it depends on two things. The type of bottom paint you use and how salty the water is. The saltier the water the greater quantity and variety of growth your likely to have. If you use an ablative paint, one that slowy wears away as the boat moves through the water, it will depend on how often you move the boat. If you rarely move it then the paint wont wear away and you'll build slime up pretty quick. Take note of boats that never seem to leave the marina and check their water lines, they'll most likely be pretty dirty. If you use a hard based paint, racers use this type for the smooth bottom it creates, then frequent scrubbing may be necessary. I don't know how clear the water is in your area but if you can see well enough you may be able to judge how dirty the bottom is. Although the more light that can get through the water the more the slime can build up. Also keep an eye on your boat speed. When she seems to be a little sluggish it's generally because the bottom is getting dirty. You'll also have an issue with barnacles in salt water. They'll cling to any unpainted surfaces and need to be scraped off wearing thick heavy gloves. They can cut like a razor and infect you very easily. They also provide a lot of drag on the boat costing you speed. Bottom paint is a science all to itself and what to use varies with how you use your boat and what type of water you keep it in. You'll likely get a variety of answers and opinions on it. Talk to a couple of local and well respected marina and ask them what they recommend. They'll know the local conditions and the best solutions. In both cases, zinc and bottom cleaning, try to be there when they do it. If you can then you'll be able to judge just how bad the zinc was and how dirty the bottom is. You can adjust your schedule from there.
