Just two weeks ago, I hiked to an overnight destination, 22 miles round trip, and I started out well hydrated (sloshing, really) with 4 liters of water in my pack because I was not sure of the water source at my destination. Total pack weight was about 3 pounds, a tad heavy, but not unreasonable. I was correct in carrying that amount; no supplementary water was available. I returned with one half liter remaining. Conditions for this trip were ideal - cool and pleasant.

I have often gone out in warmer conditions (southern Arizona deserts on SAR operations) carrying 1.5 to 2 gallons of water when I expected to reach a reliable water source by sundown. You carry what you must to survive in the ambient conditions. At least with water your carry weight declines during the excursion.