A tomato plant needs to receive at least one inch of rain or water per week for a steady rate of growth; this can be rationed into daily amounts. In warmer, drier climates, up to two inches are necessary. Water plants early in the morning to not lose water to evaporation due to the hotter temperatures of the day.
It is important to not overwater a tomato plant. This can cause the roots to not receive the air they need. If the plant looks wilted, more water is needed. The best way to gauge the amount of water needed, is to watch the way the plant reacts.