I have a book called Fasting and Eating for Health by Joel Fuhrman MD (ISBN 0-312-18719-X). In it, he recommends fasting for between 10 - 15 days around once every 10 years.

I have actually completed a 10 day water only fast a few months ago (good use of lockdown), it takes my body around 4 to 5 days to switch completely to ketosis, then to the end of the fast I was burning through 1 pound/~0.5 kg of fat per day. You can watch it happen every morning when you weigh yourself.