If I sent someone a friend request, and they haven't accepted nor declined even if they're still posting, should I just cancel the request?

One point, you asked this in the YMessenger, which no longer exists. You will not hear back if that is what you did. On your phone messenger give it a week, delete the request, and do it again. You really can not be sure they got it.

No, leave it to her to accept or decline. If it's still open after a couple of weeks, then cancel it cuz she's obviously just ignoring it. She may get a lot of requests and lets them accumulate til she decides it's time to clean them out.