Out of interest, how long should I wait or is it a case of waiting for a response?
Well, legally they have 56 days from receipt to respond, so you'll have to wait for a bit I'm afraid.
Keeping an eye on the amount should give you a clue, if they re-offer the discount you'll see the outstanding penalty go back down to the 50% rate when the rejection is issued. You can then call the council and ask them to email you a copy, which might be quicker than waiting for the post.