Dry mouth is not the only indicator of a poorly hydrated body. Often the sign can be a head ache, minor stomach cramps, nervous feeling or joint pain.
Our bodies need a constant supply of water to maintain adequate levels, since we loose on average 1-1.5 litres per day breathing, sweating and peeing. If levels remain chronically low one key sign might be joint pain. When inadequate water levels continue over time we develop an inability to remove metabolic waste accumulating in joints and muscles. This waste is very acidic with a pH level way too low for our bodies. This acid build up will irritate free nerve ending which interpret this as painful sore joints. Structurally there is nothing wrong with the joint and it shouldn't cause pain. So often increasing water hydration for a few days can eliminate joint pains.
There is now straight forward answer to that question since demand is governed by a persons size, activity level, diet and the climate they live in. A good rule is to ensure regular water hydration breaks throughout the day while avoiding drinks with high fructose corn syrup (sugar) which might quench a thirst but will not hydrate your cells.
Water is always a better choice when thinking about hydration. Green tea works very well too and will supply lots of antioxidants and a nice taste.