The validity of the PCC is one year from the date of issue.
It has to be valid on the date of the grant also
So if you do it too early, you may be required to submit a fresh one, if the invite or the grant is delayed
You can get it done whenever you want although I am told that some PSK ask for the invite before issuing the same.
So you may like to recheck with your local PSK, if you plan on getting in done before the invite