The correct answer to this question would be as soon as you can. If you haven’t started the preparation yet, now would be the right time to start. Coordinate your self-study with what the teacher is teaching. If your teacher has taught you a chapter today, go home and read it once again. Reading the same content twice a day will consolidate your understanding of the chapter. The sooner you have completed multiple readings of the textbook, the sooner you can start your revision.