I marked a book RWAP and haven't heard back from the sender. Technically there's no real reason she needs to contact me, since I told her I didn't want my credit back. But I feel like I should wait for some kind of acknowledgement before marking it resolved.

Well, she may feel that she doesn't need to reply as you told her that you don't want your credit back. So, technically, your issue with her is resolved.

 If I were you I would probably just mark it resolved. I usually do immediately after I send a PM, in the case where I am not asking for anything back.

PBS suggests giving a week to wait for a reply to a PM, though. So, that is the standard time.