What is the average time a human survives without water?
As others have stated, 3 days on average. There are several variables, however, and is not a guarantee that a person would last even that long, while others may go longer without.
One thing to realize is that in nearly every case of people being rescued from their situation, they were dehydrated before they were effectively IN a survival situation.
This significantly decreases that "3 day rule".
Other factors would include a person's health & fitness, if they have an illness, i.e., cold or flu virus, temperatures, geographic location, etc.
Three days. Depends on the environment.
The number of 3 applies in general with average humans. 3 minutes without air, 3 days without water, and 3 weeks without food. Naturally this ratio changes when a person is doing very strenuous activity, or is larger, smaller, or more/less in shape.