Currently, I am on Day 8 of a 12 Day fast, being interested much more in the astounding benefits of autophagy and mitophagy than losing weight (which IS very important). Why 12?
According to Valter Longo (Professor of Biological Science and Director of the Longevity Institute at University of Southern California), and Dr. Guido Kroemer is (a professor at the University of Paris Descartes and an expert in immunology, cancer biology, aging, and autophagy and one of the most highly cited authors in the field of cell biology) it takes 3-5 days to get into full autophagy, with the variations depending on how adapted you are to fasting, your physiology, your preceding diet, etc., etc. etc.
I use the more conservative number of 5 days, then add 7 days for a substantial whack of autophagy and mitophagy, after which the body begins rebuilding cells, mitochondria, DNA, etc., etc.
If I stopped at 5 days, it seems like I’m quitting just as I really am getting started!