There's a few ways you can do this, this one springs to mind first:
Create a schedule (once a day at some given time) and a process plan. The process plan executes on ADAccount and runs on this schedule. The WHERE clause here will select all ADSAccount which are due to expire exactly 45 days from the system date. This selects all the ADSAccount objects. Your process plan can trigger a custom event.
Finally, create a new process on ADSAccount which runs off the back of this custom event and emails all UID_ADSAccountManager.
