I use a few tablespoons per gallon (or 4 cups (minimum) per 55 gallon drum). Usually, I just throw a splash in their water dish. If you use ACV use plastic only since ACV causes reaction with the metal that can make the chickens ill. The benefits is that ACV with mother is HIGH in nutrients and vitamins and it helps prevent (and treat) sour crop (which is a fungal infection in the crop).
ETA: ACV also helps reduce (or completely prevent depending on the amount of ACV) the algae in the water dish.