How long does it takes for my refund to go back on my card after it was approved

Could be up to 30 days, but in my experience it is 3-5 business days. Once approved it is up to the bank of the credit card to post the refund to your account. If it has been longer than 30 days, then Apple recommends to contact the bank. Review the information provided here:

If your refund is approved, the funds are 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 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.
• Credit and debit cards, Apple Pay, Apple Cash, and all other payment methods—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.

Check the status of a refund for apps or content that you bought from Apple - Apple Support