Hello student student there's no fix time its totally vary student to student and on the way they study organic chemistry is very interested but also bit difficult to understand for some students so if you focus what your teacher taught in class and do a revision by yourself you'll help yourself to clear your concept.
If I talk about the syllabus of class 12 then atleast 3month is needed to complete the syllabus smoothly.

Make sure you complete the syllabus and revise the whole syllabus before the exam

Feel free to comment if you have any doubt
Good luck