Yes. You can use the number of study hours per week if that's how the weight of your grade is determined in relation to other grades. The best way is to use the number of hours specified on the transcript. If there are no hours specified on the transcript, it is generally assumed that all grades have the same weight.
For example, you can put 1 credit for each class, 3 credits for each class, or 20 hours for each class. The resulting GPA is going to be the same.