I think a gallon of water PLUS whatever other liquids are in your daily food/shake intake. Those foods and protein need water to digest and a lot more than what the food and protein shakes provide. EDIT: I just read that aloud and it sounded kind of confusing, hope it makes sense

I seen a artical where Branch Warren was talking about people try to drink a gallon of water food take into account the water in their preworkout, shakes and even food. If you drink a gallon of water then great and if you are eating foods that have a high water content such as fruits like kiwi and apples then I'm sure it can't hurt. At least you know that your muscles are hydrated and so is the rest of your body.