The correct answer is “15 years”.
Article 343 of the Indian Constitution stipulates that Hindi in the Devanagari script would be the official language of the Union, but it allowed the continued use of English for official purposes for a transitional period of 15 years from the date of commencement of the Constitution (i.e., until January 26, 1965).
Article 344 recognized 14 regional languages and established an Official Languages Commission to promote Hindi. The Official Languages Act of 1963 extended the use of English beyond the initial 15-year period.