This obviously depends on the person, subjects like science if you have a good memory you could probably memorise the course the night before. I didn't pay attention in english and managed to get AA in lit and lang just from youtube the night before.
This isn't recommended, however looking back its absolutely possible to get A* in every subject barring languages with revision, starting now.
Spent about 30 hours on each of the sciences and ended up with A* in triple science, not too difficult just do practice.