They’re drinking water with electrolytes and having their vitals checked twice a day. If their vitals start to become weak or erratic, I’m sure they’ll a taken to a hospital.

But if they actually intend to see it through, they can go for quite a while. If they also take vitamins and a potassium supplement for heart function, they could survive without food for over a year. (The actual length depends on many factors, of course, particularly starting weight.)

Without supplements or electrolytes, a person can survive for around two months. (Again, depending on starting weight and other factors.)