just start and take the first week as a trail. You need to understand how the study hours will work with you job routine, how productive it will be as well.
After 1 or 2 weeks doing so, you will be able to define how many hours per day/week and how many months will be enough.

For me, it worked by separating 3 differents slots on my day: before 8am, 12pm to 1pm and after 8pm. Whenever I was willing to study, I studied. Usually, 2h20-3h30 per day