Subscriptions last as long as you establish. Apple owns the timeframe. That said, it is indeed a timeframe. A 3-month subscription bought June 1 will expire Sept 1, and any document therein will appear as entitled.
A "your mileage may vary" hack that some publishers have done is to drastically pre-date or post-date folios. This will cause them to appear at the top (post-date) or bottom (pre-date) of Viewer App's Library (new == first), as you choose, and they're guaranteed to fall out of your subscription range. A folio with a publish date of 2014, for example, would not appear as entitled to those with any subscription smaller than 2 years or so. This really only works for limited-run publications, because they'll eventually become available when they fall into a subscriber's range.
My suggestion is a hack where you're gaming Apple's date-driven subscription logic. I'd test it out in a development/sandbox app before bestowing it on users.
