BASE ON MY PERSONAL EXPERIENCE, I honestly do not think that 15hours of studying per week will be sufficient for papers F2 and F3. However, for F1 it will be enough. I have some background in accounting so i cleared all of them with 70+. I will also like to mention that a revision kit is really important and you should do all the questions in it for higher chance of passing the exam. Approximately 22hours/week for F2 and 25hours/week for F3 as F3 contains more content in it.