I usually get Informed Delivery emails around 715am, if there's mail, and if it was scanned. Sometimes there's mail that wasn't previewed in the email.
I recently learned you can check the USPS website's Informed Delivery the night before and it'll show the same mail images in the next morning's email. This is great since I've been itching waiting for a few things in the mail.
Edit: I learned the next day's mail preview images on the USPS website are available as early as around 720pm or so. I think one key mail cut off time is 6pm, so maybe that's when they scan outgoing mail.