You should check with the American College of Sports Medicine at the following link:
This article is more geared toward professional athletes, which you may be one of them! =)
In general, I have always made sure that I drink about 16-24oz of water over an hour or 30 minutes before I exercise, drink when I’m thirsty while I’m exercising, and drink about 16oz of water after a cool-down. General rule of thumb is that you want to replace what you’re losing. exercising with limited water and then weighing yourself afterwards can be a good way of finding out how much water you lose through sweat (16 fl oz is roughly 16 oz, or one pound). However, you run the risk of becoming dehydrated if you don’t use your common sense when doing this (i.e. drinking when you need it).