You are supposed to mark books when they are received. There is no formal rule about exact timelines, though. It's just good form to mark them received as soon as you are able.

I personally don't go running upstairs to my computer the second the mail arrives. I generally mark things received the day I collect the mail, usually every day, but sometimes the day after. Or maybe the day after that, if I haven't collected the mail in a day or 2. And I travel for business often, which means that frequently I mark things received on Friday or Sat that might have arrived earlier in the week.

I once had a book scanned as Delivered to me, and it showed up in my actual mailbox 16 days later. No clue where it was all that time.

I would be annoyed at a PM received the same day, too.  Was it a new member? They tend to be the most anxious about things being marked received, I've noticed. I might have sent a reply back saying that members will generally be annoyed at receiving reminders to mark books received before they are actually received. Generally, most people who want to send a PM, wait a week or so before doing so, to allow for people who don't get their mail everyday, people who can't access computers every day, people on vacation, etc.

Also, people on the East Coast pretty much don't seem to remember the time difference ... if you are sending a PM to someone when you get home from work ... the person you are messaging on the West Coast hasn't come home yet and didn't check their mail.

Oftentimes, people do not realize that even though the DC may say the package was delivered, it could have been delivered:

1)  To the wrong address

2)  To the receiver's post office, yet to be delivered

3)  Marked as delivered, but is still in the post office truck.

In answer to your question, this is  from the Help Center:

It is important to mark a book received as promptly as possible after you receive it, so that the sending member will get credit for sending it.

When I send a book and the DC says it's been delivered and it hasn't been marked as received, I usually give it a week or so and if it still hasn't been marked as received, I will send a *gently" worded PM, explaining the DC is showing the book as being delivered on this date, was it received and if so, could they please mark it as received.

Some people can be over-anxious, especially if they have had experiences with requesters who take a week or more to mark a book received.  I requested a book once, sent media mail, and 7 days after book was mailed, the sender sent me a PM demanding I mark the book recieved. (It wasn't sent with DC). I was annoyed but kind, I told her she needed to be patient, media mail books can sometimes take a while.  The book showed up 3 days later.  She apologized and said she was sick of having books marked received just before they go lost.

I always mark it received the same day it is delivered. I hate making people wait for their credit. One the other side, I usually wait 4-5 days before sending a pm asking if they actually received the book that was marked delivered. 

The guideline for receivers is to mark the book as soon as possible after receiving it. (HC doc copied in other comments)

The guideline for senders is to wait a few days after the DC scan before PMing, which is what I think you were refering to as a grace period.

You should FIRST contact the requestor using the  button on the transaction on the Books I've Mailed tab in My Account when a few days have passed after delivery.

If people are consistently anxious about having books marked received right away it would benefit them to use printable postage and get a credit as soon as the book is mailed. It costs a little more but is worth it for the peace of mind.

Yes, annoying.

I agree with Sara & Melanie's comments. 

I also try to log books in the day I receive them, but it's rude to pm without waiting several days.