This question is more difficult to answer than it seems. As a general rule in higher education, you should spend 2 hrs on homework for every hour spent in class. If you do the math for plebe year, plebes carrying 16-17 semester hours will need to spend 32-34 hours doing homework. You get 3 hrs per night of mandatory study time, other than Saturday. So, what do you think?
However, it's a bit more complex. First of all, not all students need the full two hours for every class -- because there is less work for a given class or because the topic comes more naturally to the student. Second, there is additional time available, such as free periods, afternoons after mandatory sports, and weekends. As you get more senior, academics typically become more difficult and you don't necessarily have more free time. However, you have likely become better at studying and definitely have become better at managing the time you do have.
Most mids find USNA academics very challenging. You carry more hours than many college students. The classes are hard. And, most importantly, you have so much more to do than most college students (rates, parades, mandatory athletics, standing watch, mandatory spirit activities, etc.).
However, remember that they only accept people they think can handle the academics. The key is to keep up with your coursework and seek help as soon as you need it. The first mission of the profs at USNA is teaching, not research. Thus, they are more than willing to provde EI (extra instruction) and smart mids take advantage of this opportunity.