As early as possible is the best answer to give. That is if you want to do your best and care enough about what you get.
Try to make a revision resource for each topic as you go through them rather than rushing to make them all at the end. So if flashcards are your thing make a flashcard for each main point in the topic as you go through them as because it will save you time and stress at the end.
If you are already a year in to your GCSEs don't worry you still have plenty of time as even starting a year before the exams is better than most people.
Hope that helps and answers your question.