The notable American psychologist, William James, believed that most of what we do, how we feel, and how we think, is a matter of habit. In chapter 8 of his classic text, Talks to Teachers, James describes the importance of habit in education. The way people learn and master new tasks has not changed since James's time, and I believe that it offers a strong foundation for answering the question of practice. Playing an instrument, like any ability, is a matter of forming habits through repetition. For the habitual response (mastery) to develop, one must practice regularly. This means that daily practice is necessary. Regular, daily, practice of music cannot be substituted for one marathon practice session. In other words, one, two-hour practice session the day before the lesson is not as effective as daily practice for a half-hour. I recommend that time be set aside each day for practice.
The question of the amount of daily practice time will depend on many factors including, age, desire, interest, goals, and self-discipline. For younger children I suggest a half-hour of practice per day. For older children, adolescents, and adults I recommend one-hour of practice per day. I cannot give an age here, because each child develops uniquely.
The practice time need not be executed in one sitting. Quite frequently I will practice for a half-hour in the morning and then another half-hour in the evening. Most professionals and conservatory students commit up to six hours per day to practice (two hours in the morning, two in the afternoon, and two hours in the evening).
For the younger students, two, fifteen-minute practice sessions is easier to manage, than one, half-hour sitting. Parents should be actively engaged with the practice sessions of younger and older children. By engaged, we mean supportive, enthusiastic, complementary, and interested engagement -not authoritarian and schoolmarmish. If one wants to crush a child's enthusiasm for music fast, make practicing a chore for them! Encourage daily practice by engaging the child at the instrument, not by forcing them to do it "because you said so".
The structure of what we practice, as well as how we practice, is of equal importance. For one-hour practice sessions I recommend the following. Divide your practice time into three categories: Repertoire, Technique, and Sight Reading. Repertoire includes the piece that you are learning, Sight Reading (a completely different skill) includes reading music that you have never played before, and Technique includes scales, arpeggios, and finger dexterity.
Sight Reading should not comprise more than 10 minutes of your daily practice. For 10 minutes per day, play through music you have never read before, not stopping for mistakes!
Repertoire and Technique can be divided differently depending on what is being studied that week. If one is learning their scales, then practicing scales for 30 minutes and repertoire for 20 might be wise. When preparing for a concert, one might choose to practice repertoire for 40 minutes and scales for 10.
For one hour practice, try this structure:
Technique - 20 minutes
Repertoire - 30 minutes
Sight Reading - 10 minutes