I recommend that students set a regular practice time daily, preferably 5 or 6 days per week. The amount of time one should practice will vary from student to student. One should commit to an amount of time that is realistic, rather than try to set an unrealistic expectation, even if it is only 5 minutes per day. However, the rate of progress is totally dependent on how much practice you do, so ideally one would practice for longer that 5 minutes. Between 20 and 40 minutes for teens/adults is good. For children, the experience of practice shouldn't be a drag, so whatever time works is good. The main thing is for practice to become a daily habit. Children will generally resist this, but over time most children will gain confidence in their playing from regular practice. At some point, they will probably sit and play on their own outside of practice time.