This is a strange question which came into my head today after doing a technology exam. I starte tho think about it, because I didn't do a good exam and I was a little sad...
My opinion about it, it's that the importance isn't studying ten hours the day before because it's more productive to study a little every day. I think if you do the homework every day, review notes or repeat some exercise that you do in class you can improve your marks considerably. You don't need much time, only the sufficient to review the six subjects that you have done at school this day, probably about one or two hours, no more. However the last day before an exam, study two hours more the subject that you have exam it's the time minimum to pass the exam, if you don't do this I think it's a little difficult to pass the exam for you.
In conclusion I think the secret to get good results consists in working a little every day, if you are a constant worker the marks improve a lot and you haven't problems to pass the exams.