That's a difficult question to answer because it depends upon a number of factors. For starters, do you consider the course to be difficult or somewhat easy? For example, if you are an English major and you're weak in math you may spend more hours studying for your Pre-Calculus exam than your English II exam. If you're a Biology major perhaps you could spend very little time preparing for the Biology I exam but spend an unlimited number of hours preparing for Accounting.
There's also your strength's and weaknesses. Are you good with numbers? If so, maybe you won't need to study long for your statistics, algebra and accounting classes. If not, then you'll find these courses difficult so their exams will require more preparation time.
Also, there's the professor's teaching style that has to be taken into consideration. You could have 2-3 different professors teaching the same course and using the same text book but each of them has a unique way of presenting the material. One professor may dislike giving out A grades so they make the class difficult so you in turn study harder for their exam. Another professor may be really laid back and you may not find their course difficult at all so you spend less time preparing for their exam. Yet another professor may not give an exam and will instead focus on research papers and group projects so there's no exam to study for.
So your major, strengths or weaknesses, and the professor will have an influence on how long you study for your exams.