My personal view is that more than four hours a day is unnecessary for most people if efficient practising techniques are used. Basically most pianists practise as much as they have to in order to prepare music for a deadline, and this time will be different with each individual, varying from three hours a day to ten or more. So, practice sheet music for piano time can differ enormously from pianist to pianist. All have their special problems – and in fact many players survive by working in most if not all of these areas. I have not dealt with all the other kinds of pianists, such as orchestral players, lieder, duo and chamber music specialists and repetiteurs.