I am assuming you are talking about hydroponics since it is in the hydroponic thread. Water intake depends on many factors, but the major factor is heat. If its hot as hell your plants are going to drink a shit load as their sweating it out. I have 4 plants that are using about 4 gallons a day. So a gallon a day for each plant. Those are White Widows in the 3rd week of flowering. Hope this helps.