About once per week and then a good soaking but not enough to have standing water at the base of the plant. Don't use sprinklers (overhead watering) if you can. The water on hte leaves of the plant are a primary cause of mildew. Use a soaker hose at the bottom of the plants, if possible. Don't water during the hpttest part of the day, earlier morning or later evening is best...Too much water contributes to Blossom End Rot... the tomato turns black and squishy at the bottom end and will fall from the plant...