I won't be getting drakes. I hope at least. Any drakes will be given away immediately, to protect the hens!Once a day should be sufficient... will also depend on how many and how big the pool is... some do weekly pool cleanings and that is fine... will have to see what works for you...
As for keeping ducks and chickens together, many do... I prefer to have separate coop/duck house, but mine free range together frequently... if you have drakes, you need to be aware and keep an eye out for drakes trying to breed hens... chickens do not have the same equipment as ducks and drakes can internally fatally injure a hen...
I am only getting two ducks, I have a sandbox that is fairly little as their pool and I am thinking I will probably have to clean it once a day.