Gstr2 filling for taxpayer selected return option quarterly

Dear Sir

Please tell me, I opted for quartely return filling as my turnover is less than 1.5 crore for the F.Y. 2018-19 (apr). I filled 3B return for the month of Ampril 2018. But GSTR2 (purchase) i didnt filed. When should i have to file GSTR2 (purchase detail) for the month of April, May and June 2018. It will go quartely or monthly. 

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