How often are poster presentations rejected from conferences? I've never given any sort of presentation at a conference before, but I was under the impression that poster presentations are less prestigious and also much easier to get accepted.

I submitted a late abstract in the second round call for "late breaking" posters. I will hear back supposedly by the end of the week whether the abstract has been accepted or rejected. 

The abstract is definitely on-topic for the conference (it's a very large, general applied math conference with multiple sessions in various subfields, but the poster session is for all applied math fields). My PI/advisor is very well-known within my field. We worked on the project together, although I would be the one presenting the poster if it gets accepted. She read over the abstract and helped make corrections/suggestions as well.

Just wondering generally how often a poster presentation would be rejected in this case.
In my field (Electrical Engineering), poster and oral presentations are usually accompanied by a full paper submission (or atleast an extended abstract) which is what is peer-reviewed, so the acceptance ratio of posters would be the same as that of the oral presentations.