It all depends on the temperature and prevailing weather conditions.
Last year in the really hot spell we had, i went through over four litres of water during my walk from Gerlan to Carnedd Lewellyn and back.
I was using the extra capacity Camelbak bladder, and even after consuming so much liquid, i was still dehydrated due to the high humidity and heat.
Its difficult to say how much water a certain walker will require, as our metabolic rate can vary from person to person.
Some can get away with drinking less than others, but drinking in excess of four litres of water in a single walk is not common.
I forget how warm it was, but i simply could not drink enough fluids, as the perspiration was poring off me, and i knew how important it was to drink as much fluid as possible.
Severe dehydration is a serious issue, life threatening for some with medical issues, but its difficult to say how much water someone really needs.
In really hot weather, take as much liquid as you can physically carry.