File a chargeback. Include with the chargeback with your credit card issuer. Include with it:
1. Copy of your e-ticket receipt.
2. Copy of schedule changes (showing original & new times)
3. Copy of your request for a refund.
4. Copy of F9 response.
Make sure that the package is complete (same documents should also have been submitted to DOT). 6-12 hours as not "significant" is over the top and will likely result in a sustained chargeback in your favor.