How long to practice the piano depends on each student`s age and level.
For children up to 9 years old, 20 Mins. every day should be good. At the beginning it is recommanded that the practice to be supervised by the parents. After a while, when the child gets the routine and understand the way should practice, can do it alone. The goal is for the child to be able to practice and perform alone.
Teenagers and adults (beginners) should practice at least 30 Mins. per day and no more than 1 hour per day.
As soon as the student get advanced, the difficulty of the songs requires automatically more practice time.
It is very important not only the quantity of pracitce, but as well the quality: focus, avoid distructions, structure your practice.
Piano: For children, starting learning to play piano might vary a lot, from the circumstances to the skills of the children (finger independence, etc.). I started playing piano at the age of 4 because I had my mother and my sister already doing it and it came as something natural to me. Studies shows that the most common age when children start plearning the piano is between 6 years and 8 years old. Starting as a teenager or adult, can be also never too late. It might only take a bit more patience.
Flute: In starting learning to play the flute, both hand size and lung capacity of the child play an important role. The ideal age to start is 7 or 8 years old. After this age, you can start any time as a teenager or adult.
No matter when we start learning to play the piano or the flute, desire and love put into learning it should be a major factor to do not give up.