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I can provide a data-based answer, although ultimately it’s up to your preference and circumstance.
Generally we recommend that people start THE LATEST by the second registration deadline, i.e. mid-August.
For June exams there is a very clear start point for most candidates – January.
For December exams there is a more gradual start – some start in June, with the most starting in September (which as mentioned we think is too late).
I personally would recommend starting now, assuming you have a regular full-time job, are reasonably disciplined for study and can form a study plan (you can get one here).
You can see the data and charts here: https://www.300hours.com/heres-when-you-should-start-your-cfa-exam-studies/#.XS7ZxbLTW-o