Olivia, senders mark the book mailed, and if you buy PBS postage, you get the credit right then, when you mark it.  And another good thing about that is that even if a receiver never marks a book received, you get to keep the credit:P  Otherwise, for just the PBS DC, it's when it gets a scan from the USPS and that scan is transmitted to PBS, or when the book is marked received for anything you just print or hand write the mailing label for.

Here a link about not getting Marked Received

I always use PBS printable postage or DC because I can't stand the suspense of waiting for the book to be marked received;)

I use DC, and buy postage at USPS.  Since the book is scanned USPS at the counter, the info is downloaded in the evening around midnight & the credit posted to my account at that time.

Because sometimes RL happens and receivers forget to log books in promptly, or even the same week.

All of the above posters answered correctly, but I thought I'd summarize to make things easier.

No additional postage services purchased (free PBS Wrapper):  You receive your credit when the Requesting member receives the book and marks it as received in the system

PBS Delivery Confirmation used :  You receive your credit when USPS scans the package as delivered.  (This may end up being earlier than using the "free" wrapper method, but sometimes is the same amount of time depending on how late USPS scans and uploads the scanned information.  However, PBS DC often allows you to follow, or track, the package, en route if it happens to be scanned several times along the way.  And using PBS DC gives you a guaranteed credit, even if the package is never delivered or gets lost.  NOTE: Purchasing Delivery Confirmation from the Post Office will get you tracking/delivery confirmation scans, but does not guarantee you a credit upon delivery scan, nor guarantee your credit if book goes lost. Only PBS DC will give you that.  And PBS DC is cheaper than USPS DC.)

PBS Preprinted Postage used:  You receive your credit as soon as you click the Mailed button after you've printed the wrapper.  This is the fastest method to get your credit.  There is a built-in PBS DC, so you get the tracking and delivery confirmation benefits as well.

ta