The same thing that happens when you make a purchase on a Friday: It most likely gets posted to your account the following Monday. No clearing (or at least settlement) on the weekend goes both ways.
When I make a purchase I don't get to decide when to run the batch job for it to post/settle. Retailers however can determine when they want to actually process refunds. What can a customer complain about when the posted refund shows up on the following Monday, Tuesday, Friday, two weeks later?
To further my point of retailers not being incentivized to process your refund quickly there was a small study done that delaying refunds had customers "re-spending" the refund before it had fulled settled.[2]
[1] https://www.freeprivacypolicy.com/blog/return-refund-us-laws...
[2] https://hbr.org/2023/06/how-retailers-can-capitalize-on-the-...