Wow, you plucked some of your questions right out of my head! ("What are your thoughts on water fasting, once a certain level of health has been achieved? what do you see as the optimal, in a year? And at what frequency?")
I've read of people fasting 3 days out of ever week; one week out of every month; and so on... where fasting is a cyclical part of their regular eating patterns.
All I've known most of my life is that the less i eat, the better i feel. To address your question with a non-answer: i don't know what's optimal for me. I'm in the process of figuring it out. (Maybe 10 days every month or two?)