40 days with water and vitamins but no food is about the limit of what a healthy adult is capable of. Those who are overweight can go longer. In the extreme case, one morbidly obese man got himself down to a health weight by not eating for a little over a year. Regardless, extended fasting should only be done under a doctor’s supervision, so that you resume eating before your weight gets dangerously low, and to make sure you maintain healthy blood chemistry. Tragically, several people survived the Holocaust only to eat themselves to death when Allied soldiers let them gorge on their rations, not understanding the importance of mineral balance when resuming adequate nutrition after a period of starvation.