The frequency and amount to water depend on a lot of factors. Weather conditions and soil type are the two most important ones. Plants growing in sandy soils (or in containers) need watering more frequently than plants growing in loam or clay. Also, plants need more water when temperatures are high and humidity is low. Generally, tomatoes need the equivalent of an inch of water per week, either through natural rainfall or irrigation. It's usually best to apply it every few days rather than daily.