I'm guessing no one has ever died of dehydration on the AT. IF you are trying to go ultralight, I would try to carry no more than One liter of water. During droughts, or perhaps in PA, you might want to carry more. I see no reason to ever carry more than 2 liters. I tried to plan it so I would be out of water 15-30 minutes before the next source. That being said I did have to do an uncomfortable 10 miles with no water in July in PA once and 5 miles another time. I figure in 130 days of hiking that's not too bad. The 10 miles with no water took a toll on the body, even after drinking 3 liters when reaching water. The next day I was definitely dragging.