Keep in mind that aside from the requirements being different (RPCV or years of experience), so are the benefits...so if you are doing it for NCE or other purposes that require the "RPCV" status, PC Response positions don't give that, unless you do a Response assignment for two consecutive years, or do Response directly from regular PC service....
As for your question, the whole process is basically a waiting game - some lucky few don't have to wait that long, others do. You may have heard this before, but it really is a "it depends" situation. Some people are interviewed right away, others wait until the last minute right before the know-by date.
Some, unfortunately are not interviewed at all. This is not that uncommon, and many people now go through the process two or three times before finally getting that invite!
That being said, keep volunteering and gaining experience, and make sure you are letting the PO know when you do something new. It really does help. Under review (in my experience) means just that - they're looking at your file. No idea if everyone under consideration gets that or not, but it wouldn't surprise me if they do, as obviously they have to consider you if they want to make a decision one way or another.
Good luck, and hopefully you'll get that interview!