User blog comment:Stormwell/Timeline of Events/@comment-3525200-20140121142237/@comment-27202257-20140122052957

I'd actually adjust that a little bit, based on info from this book. It suggests that the barrier hadn't been maintained for "at least a decade" and it broke one year before the book was written. Given that the author mentions the Lord Regent, it's fair to suggest that the barrier broke one year prior to the Empress' assassination at the earliest.