Let your body tell you. Everyone’s body and levels are different - eat balanced food and minerals and without any illness you should be ok…
on really long fasts you just know… you get sooky, sad, so itchy you want to hit someone, you get nerve pain and twitches and chest pain - which can be heart or just nerves…
but balance your electrolytes dont just slam up on one… soak your feet in epsom salts and potassium chloride (from produce shop - or potash)
Drink bi carb, and a pinch or himalayan salt, pinch calcium powder, and magnesium - and you can also use a pinch epsom salts and potash too…