Wow, some crazy answers on here! Part of me is happy to see that so many people care about their chickens so much, but changing the water twice or "a few" times a day, as one person said, is just absurd. A waste of your time and a waste of water. If you have to change the water more than once a day, something is wrong with your watering system, IMO.
If you have one of those portable container waterers, keep it where it's in the shade all day long, and refill it when it's empty, or close to empty. You can clean out the trough part of the waterer as needed by lifted it and tipping out the little bit that's in the trough by inverting it--most importantly when there is poop in it--a little dirt or straw isn't as big a deal. Putting the waterer up on a piece of wood or something, so it's slightly elevated off the floor or ground, can help to keep the chickens from soiling the water. If you have an automatic watering bowl, you can rinse it out every couple of days and scrub it occasionally as needed. You CAN just use a bowl, but the chickens will step in it, spill it, and generally make it impractical--portable waterers are designed for the same purpose, and work much better, because they are built in such a way that they are virtually impossible for a chicken to knock over, and harder for them to step in, although they still will occasionally. And they're pretty cheap, and last a long time--they're also easy enough to make.
Chickens aren't people, and they don't mind drinking out of water with a little dirt in it--or eating garbage off the ground, or pooping in their house. They'll drink out of puddles and crap in their food tray, for God's sake. Even the occasional bit of green algae on the inside of the waterer isn't going to hurt them, although it's not a bad idea to scrub it out a bit as needed when you change the water. You don't need to bleach, that stuff just does more harm than good and is unnecessary. If it gets really bad, leaving it out to dry in the sun will kill the algae, or use a bit of vinegar too. I've always believed in treating animals well, but treating them AS ANIMALS. There is a middle ground here: I hate to see animals neglected, but at the same time "clean water for chickens" absolutely does NOT have to be equated to human sanitation standards--to do so is unhelpful to new chicken owners, and borders on neurotic IMO. Life is not "sanitary", nature is not "sanitary." And that's OK!
Anyway, pardon my ranting. Good luck!