This depends on your body size, how much you sweat, how active you are and what you eat and drink. Try weighing yourself before and after exercise to see how much weight you lose. Liters of water per pound lost. Monitor the volume and color of your urine to ensure you're getting enough fluid. However, to be truly isotonic, a product must replace lost body fluids without causing either swelling or shrinkage of the cells.
Sports drinks do more to enhance calorie intake than anything.
Replacing sweat losses of sodium, potassium and other electrolytes is probably a minor concern, unless you are competing in something like a triathlon. Drinking enough fluids is an important part of a sports diet because adequate hydration can help your sports performance and prevent the medical dangers associated with heat stress. The easiest way to tell if you have had enough to drink is to monitor your urine. But the recreational exerciser whose workout is of low intensity and lasts less than an hour will sweat minimally and obviously will not have to monitor fluid replacement as closely as the highly competitive athlete who sweats hard for 1 to 3 hours at a time. If you participate in a low-intensity exercise program, such as walking 1 or 2 miles, enjoying a gentle bike ride for 30 minutes, or playing recreational volleyball for 30 to 45 minutes, you are unlikely to become dehydrated because of excessive sweating.