That depends on your capacity. However, please remember that time is needed for certain essential activities like sleeping, eating, bath/toilet activities and exercise. I would not recommend more than 8 hours a day. Remember, the more you red, the more you need to rest/ sleep to assimilate what you have read. Good food and good sleep and good exercise is a must for good study. If you are studying regularly, 2 or 3 hours daily should keep you in top form.