Short answer: there isn’t a proven “X days to strong autophagy” number in humans. What we do have are a few human data points and a lot of animal/indirect evidence:

Bottom line

• No consensus human timeline for when “strong autophagy” begins across organs.
• The best evidence shows some immune-cell autophagy by ~24 h, while longer or repeated fasts likely amplify signals—but this remains unquantified in most human tissues. Taylor & Francis OnlineResearchGate
• Practical advice for seniors (safety-first)
• If you’re curious about autophagy benefits, start with gentle time-restricted eating (12–14 h overnight) and layer in light exercise—both stimulate similar pathways safely. New England Journal of Medicine
• Avoid unsupervised 48–72 h fasts, especially if you use glucose-lowering, blood-pressure, or thyroid medications; risk of hypoglycemia and other complications increases with prolonged fasting. (Standard endocrine guidance for supervised diagnostic 72-h fasts underscores the monitoring required.) MFT