"Over 72 hours is dangerous as your body starts to consume protein -muscles-."
Do you have any literature on this? I have actually been told that a minimum of 3 days is required to gain the physiological benefits. That the part of our bodies it starts to "consume" is the less-than-optimal cells, more or less 'cleaning up' our bodies. I have around 8-10% bodyfat and have fasted for up to 2 weeks and lost some weight but just a few days after eating again my bodyfat was down but my muscle mass stayed the same. I actually increased in strength a few days after ending a fast than before I began my fast. I have done 2, 2-week fasts and several 3-5 day fasts. Same phenomenon. I realize what I'm saying is anecdotal which is why I'd like to compare if you have any references!
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.