It depends of the state of the feet. But I am used to do it everyday. Horse's feet are the most important part of the body to me so I really take care of them. According to the horse, I apply hoof moisturizer or dry ointment. It also depends of the period of the year. In the summer time, my horses leave outside so I have to adapt the cares.
In the summer time I do it every two days but in the winter time it's not necessary for my horse :) the type on ground he lives on and his diary activities have an impact on the choice of an ointment and it's application :)