As much as she wants. Right now, my WW is drinking 4-5 gallons every 2-2.5 days. For comparison, root ball is nearly filling the entire 10" deep section of a 5gallon bucket.
Mark your rez and just keep adding fresh water topping it off and keeping it at that level. If the nutrient changes by >200 you might have a problem. if it changes around 50-100/week that seems to be strong growth intake.