It is a rolling basis in a way... dependent on the candidates WCS. I have seen candidates with no LOA, only a Prez. nom get an appointment in Nov, and I have also seen candidates with no LOA and no other nom. get an appointment during Mass Mailing March.
Remember there is no limit to the number of Presidential nominations, but there is a limit to how many can be charged.
If I was to guess, my guess would be that there is at least a handful with Presidential nominations that have been appointed already. Not a lot, but at least a handful.