I think it varies on a person's own biochemistry, but I think there were some papers that say 48-72 hours in, the process of autophagy gets kicked in.

There's alot of science behind it and multiple ways to do it.

There's intermittent fasting, then there's One Meal A Day (OMAD), there's strict water fasting + vitamins/minerals, there's dry fasting.

There's alot to it, but it's something you'll have to do your own research on to really dig into that rabbit hole.