I asked the same question when I was in your shoes. The truth of the matter is simply that, there's no fixed 'hours' of studying that'll fetch you good grades. I had a friend that'll sit down for hours at her desk and won't recall anything tangible 3 pages away on her note.
Ask yourself what style works for you. Do you find it easier to study in isolation? Do you find that a long chain of studying gets tedious? Does learning visually stimulate better results? Try different things and come up with a good conclusion within a week. Then stick to it and maximize its full potential.
Have you got yours?