To be well hydrated, the average sedentary adult man must consume at least 2,900 mL (12 c) fluid per day, and the average sedentary adult woman at least 2,200 mL (9 c) fluid per day, in the form of non caffeinated, non alcoholic beverages, soups, and foods. Solid foods contribute approximately 1,000 mL (4 c) water, with an additional 250 mL (1 c) coming from the water of oxidation.
So for sedentary individuals, you should shoot for about 7 cups of water per day if consuming near your calorie needs.
Base your water intake on your climate, sweat rates, and your activity levels.
You must drink like 1500 to 2500 mLs. 1000 mLs is 1L. And a gallon is... you know the rest.