A human being can go without food for three weeks on average. The longest known fast is 73 days during the 1981 Irish hunger strike. Water is more essential to life; normally, we can only go for four days without drinking it. Some people have survived for a week without water, so it is technically possible that Madeline could have survived. Note that she was put into the family vault, which would have been a very cool place. The cooler the temperature, the less water you need. In addition, Madeline suffers from catalepsy, which slows down bodily functions.
Perhaps the major consideration for Madeline Usher’s survival is that this is a tale of the supernatural. Roderick believes that the house is alive, haunted, and cursed. Madeline might have been possessed by a spirit.