According to WebMD, one needs about 8 oz. (1 cup) of water for every 15 minutes of exercise, and ideally, one should also hydrate ahead of time. This simple hydration calculator is available online to help you determine how much water you need.
If you’re taking a leisurely walk in mild temperatures, you’ll probably be all right with less water. If you’re doing strenuous climbing out in the heat, you probably need more than the recommended amount.
Sweating can create such water loss that the person will actually lose weight; it’s ideal to hydrate enough to offset this weight loss. Also, burning calories requires water, so hydration is critical to keeping your energy level optimal, even in cold weather.