Mine changed to preparing to dispatch within a day or so of the replacement being requested and were with me 3-5 days later.
Did you not get the option to keep your defective phone until the replacement arrived?
Not explicitly, but the return request email said I have 14 days to return the first phone. It is still pristine, but I'm worried if I keep using it I might cause some damage resulting in Apple refusing the return, so it is already boxed up with collection booked early next week. I have sold my 4s, so I'm back to using my old 1st gen 8GB iPhone for now!
I will phone the online store again tomorrow check what is happening with the new order.