According to Goods and Service act 2017 there are mandatory o file GST return monthly as GSTR1 and GSTR 3B BUT FROM JAN,2021 you can file now quarterly return GSTR 1 and GSTR 3B if annual turnover is up 5 Crore, for more detail you can see the circular no.143/13/2020- GST dated 10th November 2020