A bit disturbing that they haven't given us an announcement on it, considering the major nature of the change.
I just ran the question by Eddie at Live Support. He says the change will take effect at the point that eBay implements the change.
From what I gather, apparently the moment that eBay switches their coding, it will trigger a corresponding change in the Auctiva system.
So I guess the solution for those of us who wanted to prepare a bunch of longer listings today, is to go ahead and prep them on Auctiva as usual, but wait on scheduling them to post, until AFTER the switch takes effect - then do a quick round of clicking through and switching the durations, then schedule/post.