How long does it take for a refund to post to my customer's account?

Stripe submits refunds to your customer’s bank immediately, but depending on the bank processing time, it can take anywhere from 5-10 business days to show up on your customer’s bank account.

In some cases, the refund may be processed as a reversal, meaning the original payment will simply disappear from the account statement and the balance will reflect as though the charge never occurred.

If your customer doesn’t see the refund after ten business days, and they are still seeing the original charge on their bank statement, they should reach out to their bank for more information. If there is further difficulty in processing the refund, reach out to Stripe Support.

Source: https://support.stripe.com/questions/how-long-refunds-take-to-process