I am assuming that this question is being asked because it is either a debit card or you do not the available credit card to use. Just like a purchase returns, or credits, are processed the same way. They will get a authorization for the refund, and process nightly back. If it is a credit card it is up to their policy when they return the funds to the available credit. Larger banks usually 1-2 days, however I have seen credit unions take up to a week.
Debit cards are totally different issue and can really vary. I would check with you bank.