
Until last Christmas I was a Customer Services Team Leader for a well known catalogue shop ;).
We always advised people who had refunds that it takes 7 seconds to take the money from your account but 7 days to get it back. It could be quicker than that but it was dependant upon the individual banks concerned but it is never any longer.
Like Sandrah said, it is processed straightaway by the shop and it is then up to the bank as to how long it takes to be credited to your account. It is usually held in a clearing/transaction account in the meantime.
HTH
Kerri xx :)