PBS FIFO moves books to the 'lost' catagory after 26 days (maybe a few more if the zip is for Alaska, Hawaii, or APO/FPO).

PBS guidelines ask that members log in received books promptly.  So 24 hours?  Maybe 48 if there's circumstances.  If PBS DC is used, I think TPTB recommend waiting a week after it's scanned delivered.  Keep in mind the automated PBS system will also be sending reminders to the requester.

If you sent the book with PBS-DC and it has been recorded as delivered by USPS:

• If the requestor has not marked the book received by a week after the delivery scan, contact us.  We will look into it.
• You will already have gotten your credit when the barcode was scanned by USPS.

That said, I have sent PMs, worded to reassure that the book was mailed on time. You want to be careful to not sound accusing on any messages about books not being marked as received. From either side. There is nothing quite like getting PMs about stealing their credit when you actually have not received the book.

The receiver should mark a book received within a few days of receiving it, the average transit is 9 days. Of course with it being the holiday season packages won't be travelling that quickly.

If you look at the transaction, there will be a System Action Date listed in the middle towards the bottom. The lost date will be 2 days prior to that. The site gives books 26 days to travel (35 days for military and non-continental US destinations) because Media Mail sometimes can take that long. If the book arrives after that, the receiver can still mark the book received through their Transaction Archive.

Real life example: book marked  mailed on 9 NOV was declared lost on 5 DEC and my credit was returned. Then book got delivered late that afternoon, I marked it received in the transaction archive as it no longer appeared on "awaiting delivery" tab. Then PBS gave credit to the sender.