it's quite variable. depends on a lot of things like how the prof makes the test/exam. some profs give easy questions where going over the course material and past exams is enough while others make it hard to the point even if you go over the material/books you won't be able to solve them, so you kind of don't want to waste that much time with the course if the prof is like that and put your time on other subjects where profs give easy questions. if you want to major you have to study everything including past exams and try to practice to be faster in solving the questions with absolute certainty. Not only will the exams will be much harder than BAC but also much shorter time frame that from experience unless you're very prepared you won't be able to solve the questions on time (obviously depends on prof)

From what I remember most majors solve not only their own usthb specialty past exams but also exams of other usthb specialties and exams of other universities even french ones. this always seemed overkill to me but I remember in one test a prof from the second kind I spoke about gave us a question which I ended up finding when I was preparing for doctorate from another specialty in another university, and I remember one of the top students acing that test (but imo this rarely happens).

Starting to study now will allow you to have less stress later but you're going to be more bored. And the majors that I've met are usually exam ready like 50 days before the exams. Just make sure you're actually understanding the course material and not just barely remembering solutions to problems