As per this University of Missouri info, figure at least 1 cup of water per day per laying hen..... more to allow for any leaks or spillage. That is winter levels. Allow for double that during hot months of summer.
Do not overlook the "fresh" nature of the water we give them. A lot of containers will pick up off flavors or the water will go stale. If you have ever sipped water that has sat stale in a standard rubber garden hose for several days, you would understand. If you want them to drink enough, the fresher the water the better.
Water
Do not overlook water as an important nutrient. A constant supply of clean, fresh water is essential to maintaining healthy poultry. Twenty hens can drink about a gallon of water each day in cool weather. Water consumption increases dramatically during hot weather. Thoroughly clean water fountains or troughs once a day or more often if necessary. Refill with a clean, fresh supply of water. Be careful not to spill or empty water on litter.
Provide clean, pure water for birds at all times. Increase the supply to 3 gallons per day for each 100 chicks at six weeks of age; to 5 gallons at 10 weeks, and to 6 gallons at 12 or 13 weeks of age. Supply 6 to 8 gallons of water daily for each 100 layers. Provide additional water when temperatures are above 80 degrees; for example, at temperatures above this level, 100 layers require at least 9 gallons of water per day.