There are a few issues to fix here.
It sounds like we should be sending the notifications if there are users who have been waiting for approval for X hours or longer. Even if I add a setting for how often the notifications are sent, you might get notified for someone who signed up 1 second before the scheduled notification.
There’s also the problem of repeatedly notifying about the same list of pending users. Notifications should only be sent if there are new users waiting to be approved.