hippy is right. a healthy adult can go without eating for a month or longer, but he has to drink at least 2 quarts of water a day. 11 days at a temp of 60F is the longest recorded time a human survived without water. if you lose 12% of your body weight in sweat you die, and if you lose 20% of your weight in water, that's imminent death.
a personal note- i had my jaws wired for at least a month, it was ages ago, so i don't remember exactly how long; was living on milk and ensure, it's an adult formula, donnow if you folks have it in England, and i lost about 20 pounds, i'm only 105. i was a skeleton. so, i do beleive that you can go on for the month on water only.