Hyde here from Shopify. Great question!
A refund takes up to 10 business days to be credited back to your customer. We have more information on the full process here. You can refund the entire order:
And we have a video on how to do that here:
And these are the steps here:
You can also partially refund an order:
I hope this helps, have a look through the help document and let me know if you have any more questions!
All the best, Hyde.