Generally the time period is not specified for final registration. You should have a valid final registration number at the time of filing of Final examination form with Institute.
So I will suggest you to take registration in final 3 months before filing the examination form. At that time the Institute will send your final registration letter.
As per Regulation 50 of the CA Regulations, 1988, a student will be eligible to appear in Final Exam, after completing of 30 months (2 Years and 6 months) of article ship.
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