There are different factors involved here. A human being isn’t an automobile which needs a certain amount of gas. There are conditions under which a person could die of dehydration within a single day, and other conditions whereby a person might live for over a week. It’s unlikely, though, anyone could go much beyond that. Water comes into our systems not just from drinking but from eating as well. It goes out of us through urine, through sweat, a even through exhaling. Ever notice the fog you can make on a window by breathing on it? That’s water vapor leaving your body.