First of all, the amount of time that a student needs to practice differs greatly! Below you will find some general practice guidelines. More is ALWAYS better. 
My personal best was 14 hours straight one day while attending junior high school. I was nuts!
5-6 years old > 5-10+ minutes per day, 3-5 days pers week.
(they will usually need your assistance)
7-9 years old > 15-20+ minutes per day, 3-5 days per week.
10-12 years old > 30+ minutes per day, 4-5 days per week.
13 to adult > 40+ minutes per day, 5-6 days per week.
– Comments welcome!
