I've never had a problem with refunds from Indigo in the past. They have sometimes taken over 2 weeks but have always gone back to the original mode of payment which has been an Indian based credit card.

I don't think I have ever asked for a refund for a ticket that was booked 6 months in advance so have no clue about this 6 month issue.

Unfortunately you have no other option but to keep following up. Ask for a supervisor & insist on the refund being issued to the card used to make the original payment.