Fill them with water and put them in your toilet tank, safely away from operating mechanisms. In an average home, the bottles may displace and save ten or more gallons a day. Even the smallest drip from a worn washer can waste 20 or more gallons a day. Larger leaks can waste hundreds. Leaks waste water 24 hours a day, seven days a week and often can be repaired with an inexpensive washer.
Early morning is generally better than dusk since it helps prevent growth of fungus.