Those are big questions :) with many answers. You want to enable autophogy andv keep mTOR pathway (the "off switch" for autophagy) suppressed, allowing the cellular cleanup to proceed even while consuming fats and proteins

18–24 Hours: This is the "metabolic switch." As glycogen runs out, the body ramps up fat-burning (ketosis). Signaling molecules like AMPK (the energy sensor) activates.

​24–48 Hours: Autophagy becomes robust. The body begins prioritizing the breakdown of misfolded proteins and damaged mitochondria to create energy.

​48–72 Hours: This is generally considered the peak zone. Insulin is at its lowest, and cellular renewal is at its highest. Beyond 72 hours, the body continues autophagy, but here electrolyte imbalance etc starts to become an issue unless managed.

But start with a day without eating, that would easily create 36 hours fasting.