Mine is probably not the typical answer, but it'll be one point along the bell curve when you get the others' responses. I'm also a diver and can check/change the zincs whenever I need. For #1, when I first bought the boat, I used to have to change the zincs every 2 months because of stray
current. Since a complete rewiring was done 2 years ago, I have
lost count and it seems to be more on the order of 6 months. I have two zincs on my prop shaft, so I replace one when it is about half way pitted and degraded, and before the screws are exposed. I don't think there's any way to tell (short of sending a diver or
camera down) whether a zinc needs replacing without looking at it.
For #2, I clean my boat about every 6 weeks before a big
race. I
race more often than that, but the bottom stays clean enough. I also took all the ablative
paint off in my last haulout (April 2016) and put on hard
paint, figuring that it would better hold up to my more aggressive approach to bottom
cleaning. I also only clean the bottom with a shammy cloth -- no abrasives or scraping allowed (or needed)!!
No matter what kind of answers you get here, your diver can also advise you about what is seen and how often the zincs need to be replaced. Under normal conditions, I would think it could be done every 4-6 months. You'll get much better performance out of the boat with a clean bottom, too!