Enough so you are not thirsty.
Enough so that your mouth remains wet.
Enough your performance does not suffer.
Enough you are able to maintain proper body temperature regulation.
Not so much that you need to pee more than every hour or so.
Enough that your pee is nearly clear.
"Enough" varies with environmental conditions & level of exertion. There is no one set defined measured amount that applies beyond a specific individual in a specific set of circumstances.
How much wood could a woodchuck, chuck?