It's possible to go for 15 days without any water. That is of course if you stay in the shade all the time, don't move and hardly ever eat.
The average person loses 2-3 litres of water per day. Even 1 litre is lost if nothing is done, and the person is in the shade all day.
By the time 1/5 of the water in the body is lost then you...well....you have great difficulty in staying alive. Considering water makes up most of the body, it's pretty important.
I'd say about 2 litres of water a day (that can come through drink, food, etc) and you can survive. 1.5 litres a day if you're not really doing much at all.
That's about the minimum without having a negative net water intake per day.