Wow, that really depends on a lot of things; mostly related to where I am hiking and the time of year.

When I hike in the Sierra or in the Cascades, I seldom carry more than a liter unless I anticipate a dry camp. Then, I will fill my 3-liter Platy at the last reliable water before camp. I use either a Steripen or chlorine dioxide tabs if I feel the water needs to be purified. The "Pen" will give you drinkable water in a bit over a minute.

When I hike in the Grand Canyon, I carry anywhere from a liter to two gallons. Again, it depends on water sources along the route I'm hiking and the time of year. I also carry one liter as an emergency reserve. I have used it several times; always for someone else.

When I hike the mountains of Arizona, I generally carry three liters. I know where water can be found in most of my favorite areas but like to have enough to get back to the car if a water source has dried up: and that does happen.

When I hike the desert country, I carry all the water I think I will need and a reserve liter as well. I plan on a gallon per day or more and don't travel after 10 am or before 5 pm if it is hot. Because of the weight of water my desert hikes tend to be short unless there is stone reliable water en-route.

I seldom use more than a gallon per day and dislike very much carrying an extra liter of water from one water source to the next. As noted above, I do carry reserve water in dry country and am pretty conservative about having enough water. I, too, keep a liter by my bed at night.