General guidelines are to drink 17 to 20 ounces of water 2 to 3 hours before exercising, another 8 ounces during your warm-up (or 20 to 30 minutes before exercising), 7 to 10 ounces every 10 to 20 minutes during exercise, and 8 ounces of water within 30 minutes after exercising.
Every individual is different and the amount of water needed to hydrate varies from person to person. It is important to be appropriately hydrated before commencing exercise in order to avoid getting cramp. However, avoid drinking too much or you will need to go to the toilet! While exercising, sip water at regular intervals every 20 minutes or so especially in hot weather or at high altitude. For high intensity exercise lasting more than 1 hour sports drinks
or fruit juice diluted one to one with water may help extend time until exhaustion. It is also important to rehydrate after exercise, and for most people water is adequate. This is best achieved by sipping at intervals, rather than gulping large amounts at a time.