If you are the supplier, you have to fix the credit period (15/30/45/60 days) and also the conditions stating, "if the amount is not received within specified time Interest @ 15/18%..etc should be paid".
As per Section 16 CGST Act if recepient does not make the payment of goods or Seevices to Supplier within 180 days has to reverse the ITC with interest on the same, will added to his Output Liability