If you want to give your fish the best conditions and make them grow fast and big as possible, you should do WC as much as you can. But if you only want just to keep them healthy and you don't really want to get them to the max size, even 10% per day is good enough. Maybe more if you have load of fish and plants and you can even less if it's a bare bottom and not loaded aquarium, if WC is trouble for you. You also have the right for good ang happy life, not only your fish