Depends on the subject and the persons aptitude for it. It varies. At my uni, Intro To Biology was a weed out course to deter all the pre-meds, pre-dental, & nursing students. Kids dropped like flies, but I, and a few others excelled with little to no problem with only a few hours of study per week. However, in something like Physics, I had to study almost daily and dozens of hours per week in order to attain a B. There's nothing wrong with studying for hours as long as you're getting a good grade out of it. It may mean you're incredibly proficient in the subject, but you don't get cookies for having to study less than your peers.
You say you don't think you're study efficiently. What sort of methods do you use to study? More importantly, what kind of grades are you netting, exactly?