It depends, the usual number quoted is about 2-4 days on average, but it can be as high as 12 days, or as low as a few hours.

A well-hydrated, healthy individual sitting at room temperature with no food can potentially go as long as 12 days without water (though that’s the very upper limit) – the average person would die after about 3-5 days. In high heat however, a person could only survive about 2 days – in the shade. With exposure to the a hot sun, a person with no water can experience dehydration within an hour or two, and could die within a day.