How long does a refund from App Store take to show up on my bank account?

If Apple has approved your request, see this Apple article: Check the status of a refund for apps or content that you bought from Apple

Check the status of a refund for apps or content that you've bought from Apple – Apple Support (UK)

When will I get the money back?                                            

If your refund is approved, the funds will be applied to the same payment method that you used to buy the item. 

Here's how long it takes for a refund to appear on your account or statement:

Store credit – It may take up to 48 hours to see the refund in your Apple Account balance.

Mobile phone billing – It may take up to 60 days for the statement to show the refund. Processing time may be affected by your network provider. You can contact your network provider for an update.

All other payment methods – It may take up to 30 days for the statement to show the refund. If you can't see the refund after 30 days, contact your financial institution.

Read this support article for information about how long it takes to receive a refund, and how to check the status of a refund for apps or content that you bought from Apple --> Check the status of a refund for apps or content that you bought from Apple - Apple Support

After you request a refund you'll receive a status update by email within 48 hours. If your refund is approved, the funds are applied to the same payment method that you used to buy the item. How long it takes for a refund to appear on your account or statement depends upon the payment method used:

● Store credit—It might take up to 48 hours to see the refund in your Apple Account balance.

● Mobile phone billing—It might take up to 60 days for the statement to show the refund. Processing time might be affected by your carrier. You can contact your carrier for an update.

● All other payment methods [e.g., credit or debit cards]—It might take up to 30 days for the statement to show the refund. If you don't see the refund after 30 days, contact your financial institution."

Since the article specifically says "statement", which for me is the thing that arrives monthly in my mailbox, I wonder if you check your daily card transactions online if you might see it earlier?