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The rule of thumb for health is, four 8 ounce glasses of water per day. If it is above 90 or so and one is exercising, you might use twice that much or more. Our human thirst mechanism is actually not very good, and one can be far behind water loss by the time thirst appears. My suggestion
is 6oz prior to exercising and 4-6oz every 20-30 minutes. Sports drinks are fine if they are less than 5-6% sugar, in fact, they absorb faster. Water is cheaper (1/50,000). 

A marathon runner may lose up to 10lbs (mostly water) in a race! Even in winter, indoor exercising needs more water than mentioned above.

If you are ill or take blood pressure, or any other serious medications, PLEASE
check with your doctor.  / GLD