Firstly it depends how many school subjects you have. The average time needed to study according to a recent research is 2h per 1 held school lesson. So if you have one lesson 4 times a week, you should study on average 4*2=8h per week for that subject. This is a rough estimate, i can say. According to me it depends on your efficiency, health and intelligence. I will be glad if you try to study efficiently, look on your health and be dedicated to what you do, because than you can do better.