Flood/Drain, how much does a plant drink per day?

Hi

I´m planning a new flood/drain setup and was wondering about how much nutrient/water a plant drink per day in full bloom? Need to know regarding reservoir size.

Will use 2x600w Hps

I'm no expert (still on my first grow, switched to 12/12 about a month ago), but I have six plants in a 2 28Qt plastic storage bin flood/drain system I made. The plants sit in net cups that are in holes cut through the lid of the top 28qt container; the other 28qt container is the reservoir below. The plants look like they're making a small brain with all the roots tangled together. My very unscientific method of knowing when the plants need more water is I listen for a squealing sound that the water pump makes when it sucks in air with the water. When I hear that, I add a gallon of fresh water. After I've added 3 gallons of fresh water (the system holds 6 gallons), I flush everything out and put water with plant food again.

Hope this helps. Send me a message if you want pictures of my system for whatever reason.