WoW Insider notes that the page on Battlenet promises it "on or before December 20, 2014." That's awfully specific, especially given that the release window used to be a much more broad 2014. It also solidly puts it within their fall target.
Given that pre-orders have already begun, however, it seems likely the expansion will hit sooner than that. This date gives us a deadline that Blizzard has apparently self-imposed, but it's doubtful the studio plans to wait that long if they can help it.