Yes! Though [initially, since edited my post] I incorrectly suggested there wasn't any membership time required, just post count, but I rechecked the stickies in PRSI, and it's documented at 100 posts + 1 month membership (not sure if the latter is something new, or has been recently revised).
90 days I sort of plucked out of the ether, it just seemed like enough duration where someone trying to just get into PRSI (or what I expect is the case with some recent registrations ... BACK into PRSI ...) would me more inclined to "give up".
Due to the nature of PRSI, I think it should really be "privileged access", a new user should really have to show they can be a level headed, contributing member of the forums before they're allowed to mix it up in PRSI.
As far as I recall, there wasn't a time limit requirement on PRSI when I joined (cough) almost exactly a decade ago, just the requirement that you post 100 posts (which, I suspect, was also intended as meeting a sort of time requirement - the idea being that nobody could possibly post 100 irrelevant posts in a short span of time).
However, a month strikes me as a perfectly reasonable time requirement, whereas asking members to have had forum membership for three months before access to PRSI is granted may serve as a useful deterrent to some, or incentive for others.