I don’t think it matters. But if you wait too long you’ll lose your fat adaption and it’ll be harder to start fasting again. I personally like to eat for 2 days max when doing rolling fasts, but ideally just 1 day.
Depends on your body. What are you vitamin/mineral stores like? What does your blood work say?
If you haven’t fasted a ton in the past, you can likely get away with shorter intervals of eating between your 5 day fasts, but if you fast on the regular and want to throw in more extended fasts, you’ll need more eat days in between extended fasts. Hard to say how many without knowing anything about you. Probably a better question for a healthcare professional who has seen your blood work.
If you decide not to do blood work and proceed with consistent extended fasts, just be sure to keep your re-feed days focused on very nutrient dense foods, and it would probably be worthwhile to throw a well-rounded vitamin/mineral supplement in there as well.
Edited to add: My naturopath said I could do one 5 day fast per month and suggested I monitor my blood pressure and blood glucose levels during the fast.