I would not hesitate to file a chargeback if a refund is requested and not initiated in short order (EU is 7 days, US is 7 days for air carrier and 10 for TA).

Always do the chargeback in writing (could be an email or webform) and always attach or include the appropriate documents which make for an irrefutable case.

Some countries and card issuers may have time limits. Do not let those pass or come even close.