I try to consume an ounce of fluid per mile. In honesty this is likely not enough in the heat we are currently having. As to the use of gatorade, on the rare (and I mean rare) time I use it I cut it 2:1 with water : gatorade.

Its very seldom that water is not sufficient on a typical ride (45-60 miles). If I want calories on my bike they are either gels, chews, or a peanut butter and jelly sammich.

But as far as how much, the ideas listed above about enough to have no net loss of body weight is a good goal but one I rarely am able to meet.