The most important is the ability to keep proper proportions in the day planner. Johannes Brahms used to say that we should spend as much time playing as reading.
Artur Rubinstein has more accurately defined the number of hours we should spend on piano practice, but also on most instruments. He said:
Never practice more than three or four hours a day. No one can concentrate longer than that, and you must spend the rest of your time learning about life and love and art and all the other wonderful things in the world. If a young person sits in the practice room all day, what can he possibly have to express in his music?
