I would not advise waiting until you are thirsty.
Thirst is not your body saying it WANTS water,its your body saying it NEEDS water.
It takes time for the brain (The Hypothalamus,if I remember correctly) to respond to dehydration and trigger a thirst response,prompting you to take on water.
If you wait until you are thirsty,you are already dehydrated.
There are studies showing around a 3-4% drop in hydration levels can affect athletic performance by as much as 17-18%.
A good measure is to divide your weight in kilos by 30,then that is the minimum amount you should be drinking in litres on NON-ACTIVE days.
90kg divided by 30=3
so 3 litres.
It would be more on active and/or warm days,I drink at least another 2 litres,or 250ml for every 10 mins of exercise.
Clean,pure water is ideal for the majority of this,but remember other liquids do contribute to hydration,for instance,milk is 95 water,many soft drinks/soda and fruit juice also contribute.
It has been reported that caffeineted drinks do not dehydrate the body as much as previously thought,despite the diuretic effect of caffeine,so tea,coffee,cola all contribute to a certain extent.
A good measure is to watch the colour of your urine-
it should be clear or VERY pale yellow.
Be aware if you are supplementing with creatine,hydration is even more important and you may need additional water.