There is no formula for the amount of water required for a plant. There are too many variables such as soil type, type of container, humidity, temperature, wind, bright sunshine or cloudiness, soil additives such as composts and its makeup. You just have to learn by the plants themselves when to water. Vegetables, especially tomatoes will tell you if they are in need of water. I look at my plants around 8 AM every day. Wilting in the afternoon is normal and NOT to be taken as a sign the plant needs water. If they are wilted in the AM I water them until the soil is completely saturated for the first 2 or 3 inches. In a 5 gallon bucket with my soil this takes about 1 gallon of water. You have to remember that the soil on the surface is dry an inch or two deep but the roots are probably about 4 inches deep and go even deeper, so they may not be dry, only slightly moist. My soil is light and fluffy so a gallon of water goes pretty deep. In heavier soils this changes. The most important thing is to have GOOD drainage. You want any excess water to drain away. And remember that you cannot give a plant too much water when you water. With proper drainage no matter how much you water it won't hurt the plant but if you water too often and water when a plant doesn't need water you will cause serious problems.
