How often do I need to change the chickens' water?

Water should be emptied and refilled every second day, and probably even daily when it's hot weather. Check the water daily for poop, straw and other debris that might be in the water and check also for slime; if you see any of these things, the water definitely needs changing and if it's slimy, the whole water dispenser needs cleaning. Every week, the water dispenser should be thoroughly cleaned with hot soapy water to remove germs; you can also use bleach (2 teaspoons of bleach to one gallon of water) to sanitize the water dispenser, just take not to mix the bleach with anything else and ensure it is all washed out before using for the chickens again.