Ketosis only begins when you ru out of food you just ate/digested, and have used up a lot of your liver’s glycogen stores. Your liver stores about a day to day and a half of glycogen, and it looks like it will use up at least half before it signals to start accessing fat for fuel (ketosis). [Some will say the liver uses it all up, others say the body keeps some in reserve; I tend to believe the body is inclined to keep some glycogen in reserve for survival, just like the muscles don’t “share” their glycogen but keep their supplies for fight/flight.] The point is that if you refill your glycogen stores every day by eating carbs, you will have to burn thru those glycogen every day before you enter ketosis. So, what you eat (and how much) will greatly impact how quickly you enter ketosis on a fast. Similarly, if you are active (especially things like HIIT, weight training, and even walking), you may burn through carbs you eat before they get stored as glycogen. I definitely notice that if I am active, 15g carbs may not even budge me out of ketosis, but if I am NOT active that day, it might knock me out for a couple hours (and yes, I love my Ketonix breath meter with all my nerdy heart, lol).
Autophagy is only recently a hot area of research (since 2011 or so), and Ohsumi received the Nobel prize in medicine just in 2016 for uncovering the molecular pathways of autophagy (and that was for work in yeast, not people), so this isn’t a solved problem. Ohsumi’s work has opened up a lot of new areas of study, but most is still being done in “model organisms” (ie, yeast and mice, not people yet). <– Fancy way of saying, no one really knows. lol
So, autophagy times are estimates – generally some hours after you have entered ketosis. Some say you need to only water fast, but that’s probably being conservative for folks who are fasting purely FOR autophagy – who wants to fast for days specifically for autophagy and not get the max benefit because of a little coffee? However, we know autophagy happens even outside fasting (to a much lesser degree), so anytime we are fasting and in ketosis we’re *probably* moving into enhanced autophagy even if it’s not a water fast.
TL/DR: eating low carb with a caloric deficit and being active before and during your fast should result in you entering ketosis faster because you maintain a low glycogen storage. For me (<50g/d, <20g/meal), that means I usually enter ketosis within 1-2 hours after eating, often I don’t even fully leave it. Autophagy….well, hopefully isn’t too far after that, but I plan to add some 36 hr fasts specifically for autophagy, starting when I’m already in a ketogenic state. When I first started IF and very low carb, it took a few days. If I eat higher carb for a day or two (100-200g) it’ll take me a day to return to ketosis. YMMV
Honestly, this depends on your size and activity levels and people who will tell you a time are giving you rough estimates.
If you’re tall, muscular, and have a active lifestyle you’ll reach autophagy faster than if you’re short and skinny and sedentary. I suggest buying ketone test strips to get an indication.
It depends on what you ate and were eating in the days before you started your extended fast and a few other things.
If you were eating a keto diet and already in ketosis before you started the extended fast obviously you will be in ketosis immediately. There are advantages to starting a keto diet before starting an extended fast. One of them is that you dont have any hunger during the first couple of days unlike people who aren’t in ketosis before starting a fast.
Other factors are how much food is in your system and whether you exercise. If you exercise you burn through the food in your system faster.
If you eat 2,000 calories every meal before starting your fast, your body has to process all that before you get into ketosis. How long will that take? Who knows. Could be a day or two or three.
Many people get constipated during an extended fast and then get diarrhea when they refeed. Sort of like your body doesn’t want to let go of what is in your digestive tract until there is more food coming in. If that is how your body works, you might want to consider a laxative like ‘ducolax’ or a salt flush. Basically a half-tsp of salt in a glass of water that you drink in a few swallows and then stay near a toilet for a few hours. You could google ‘colonoscopy prep’ for an idea of how the AMA types would have your clear your digestive tract. Basically it is laxatives, a gallon of electrolytes/salt solution you drink 8 oz of every 15 minutes until it is gone, lots of water and then anti-gas pills. You start at about 6 pm and are done with the diarrhea by 10 pm-ish. The following morning, those pipes are clean. If you are considering that you should consult with your doctor first.
For autophagy, which is unrelated to ketosis, I’ve read that it starts around 36 hours and peaks at 72. Google “fasting and autophagy’ or get a copy of The Complete Guide to Fasting by Jason Fung, MD.