In term Section 34(2) of CGST Act , the credit note for Goods return for goods supplied in FY can be issue till September month of Next FY .
So if suppose goods being supplied in April 2018 , the credit note for the same can issue till September 2019.
Note : You can take goods return even after September but cannot reduce you output tax liability. So in this condition it's better take Tax Invoice from Receptient for the same instead of issuing Credit note to him.