When it comes to timing and mail, it can really vary YOY, by candidates, and by SA.
If you really want the offer of appointment LETTER in hardcopy, ask your DS to download and email to you, splurge and use a color printer and put a high quality sheet of paper. If you are actually talking about the letter plus BFE appointment CERTIFICATE - the one you take pictures with - that could be anytime. 2024 DD's nom 12/13 phone call, 12/18 upload, appointment letter only in portal 12/24. BFE postmarked 01/23. In the meantime, I highly suggest getting a yard sign, make sure DS has a USAFA t-shirt/top, be ready to triathlon sprint your way to the dollar store for balloons (and recon some items on the packing list!), and then you are ready to take that announcement picture!