https://www.medicinenet.com/how_long_do_you_need_to_fast_for_autophagy/article.htm

How long do you need to fast for autophagy?
Depending on the individual’s metabolism, significant autophagy may take two to four days of fasting in humans. Autophagy is believed to begin when glucose and insulin levels drop considerably. Animal studies have shown evidence of autophagy after 24 hours of fasting, which starts peaking at around 48 hours of fasting.

Some studies have detected autophagy in human cultured neutrophils (the most abundant type of immune cell in the blood) after 24 hours. There are, however, no conclusive studies on humans that indicate an optimal period of fasting to achieve autophagy. However, do not attempt to fast to induce autophagy without discussing this method with your doctor.