So, as the thread title now shows,
they announced today that this will be a four-part OVA series, released monthly. They don't say how long each episode will be, though I suppose we might presume the normal length. They mention that it'll pick up where the anime left off with Volume 11, and the final was Volume 15, so five volumes in four episodes... well, it'll be an overview.
I'm still looking forward to seeing how the franchise ends.