Refund for purchases made through Apple Pay are the responsibility of the Store, Merchant, website or App through which the payment was made and from which the refund was requested. Apple has nothing to do with those refunds whatsoever. You go to the merchant to ask for the refund not Apple.
If the Merchant approves the refund, then it's down to your payment method issuer or bank how fast they can process it and reflect it on your payment method or bank account.
click here ➜ Get a refund for purchases made with credit or debit cards using Apple Pay - Apple Support
If you mean refunds for items purchased from the Apple iTunes and App Stores, those have nothing to do with Apple Pay whatsoever, and Apple normally takes around 3 to 5 days to process. Once approved by Apple, it's again up to your card issuer or bank to process and reflect the refund on your payment method or account. Those can take up to 30 days to reflect on your account after Apple approves them as explained the support document linked below.
click here ➜ Check the status of a refund for apps or content that you bought from Apple - Apple Support