http://www.livestrong.com/article/437499-how-long-can-i-fast-before-i-start-losing-muscle/

Autophagy starts as soon as your blood sugar levels drop to below normal, and this changes from person to person. Therefore it is very hard to find out the exact time to fast. I stay as active as possible in my first day of autophagy to burn as much glucose as possible and then prefer to not pass 72 hours.