While these aren't common, they aren't as uncommon as you think. A famous long one is https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast https://en.wikipedi

While these aren't common, they aren't as uncommon as you think. A famous long one is https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast https://en.wikipedia.org/wiki/Angus_Barbieri%27s_fast ; But from memory reading studies about it ages ago (mostly examining religious fasters in India), an average non-overweight person can easily do 40 days water fasts without any irreversible damage. Somewhere between 40-50 days, the body would start to digest the nervous system for energy and that's irreversible (and leads to death within days if the fast continues). But if fat stores are available - as in the case of Angus Barbieri above - it can go for much, much longer.