Every day, a person loses water volume depending on environmental conditions, such as weight, age, type of food and activity performed. This water must be replaced as it is lost, before the first signs of dehydration appear.
Thirst is the body's defence mechanism for us to note that dehydration has started, and usually is the first of its symptoms, but one does not have to wait to drink until feeling thirsty.
The colour of one's urine can be used as a criterion for adequate hydration. More intense colours can warn of kidney failure or other imbalances caused by dehydration.