First you’d need to go beyond the time it takes to deplete amino acid stores, that may take around 24hrs according to some estimations. Some corroborating in vitro evidence for that “The most common method for in vitro “starvation” of cells to increase autophagic flux is to culture them for several hours in Earle’s Balanced Salt solution (EBSS), which lacks essential amino acids” (Measuring Autophagy in Stressed Cells).
Furthermore (same ref), “measurement of autophagic flux depends on the ability to distinguish between autophagosome formation and completion of autophagy through lysosome fusion and degradation”. Autophagic flux is what we’re looking for – when is it highest after not eating for a while?
In this review, improved neuroapthy was correlated and tested for protein clearing mechanisms (at the heart of autophagy clearly) ==> a lot of the rodent data suggests 48hrs. now, their metabolism is way faster, so maybe it takes way more time in humans who have a slower metabolism (Dietary restriction supports peripheral nerve health by enhancing endogenous protein quality control mechanisms). On the other hand, humans naturally enter ketosis quite easily compared to dogs and especially rodents, so maybe we get some of the ketotic benefits that accompany fasted autophagy.
In Valter Longo’s recent review we may have a hint: “Recently a series of studies in animal models have shown that periodic fasting (PF) lasting 2 or more days can be as effective as chemotherapy in delaying the progression of a wide range of cancers” (Impact of intermittent fasting on health and disease processes). If it’s confirmed autophagy is primarily responsible for this vs the ketones (for instance) then maybe +2 day is about right.
My guess is from 3 days on it gets super interesting, autophagy wise or other