Fat is 3500 calories per pound. a 100 lb woman is most likely about 15% body fat. This is 52,000 calories! At 1000 cal/day, this is 50 days. Most of us have more reserve than this.
We are supposedly 70% water. a 100 lb woman carries about 70 lbs of water. This is about 9 gallons. Obviously this is not all usable, but you get the picture. If the body is conservative about using it's reserves, you may only need to use 8-16 ounces a day. This could sustain one for a long time.
I just saw a special on penguins. The male penguin sits on the egg for 65 days waiting for the female to return with food for the baby. During this time, the penguin neither eats or drinks.