We're over 60 days since we submitted the challenge (28 March) and we haven't heard anything.
The council website says this:

There are only two possible explanations:
1) The PCN has been paid, in which case the council will ignore the representations and that's the end of the matter, or
2) The representation has been accepted and the PCN has been cancelled, in which case either the notice of acceptance was lost in the post, or you'll get it at some point in the next few days.