I recently learned the “Survival Rule of Threes”–which gives a rough estimate of how long a person can live in various extreme survival conditions. For example….
…you can expect to live for 3 minutes without air.
…you can expect to live for 3 hours without shelter (in an extreme condition like Antarctica for example).
…you can expect to live for 3 days without water.
…you can expect to live for 3 weeks without food.
Obviously these are only rough estimates (and I don’t encourage you to find out whether or not you can beat these various marks) and there is a wide variability between human beings. Gandhi was able to fast for 3 weeks while already in his 70s. One member of the Irish Republican Army, Thomas McElwee, successfully survived for 73 days during a hunger strike. The infamous case of Terry Schiavo in 2005 ended with her death about 2 weeks following the court-ordered removal of her feeding tube.
Not surprisingly, one can last a lot longer without food than without water, a testament to (a) how much water one loses with just the normal processes of breathing and sweating, and (b) the potentially lethal complications of hypernatremia and the importance of maintaining appropriate water concentration.