I actually have no idea when they are going to release in that the possibilities seem uncharacteristically wide.
On the one hand, yes, it'd feel strange to see them not taking 90% of the 2018 to get the thing out the door. They have never been fast, hard to expect surprises. That suggests end of 2018 - like October / November, maaaaaaaybe September.
But on the other hand with BFA containing barely anything worth talking about apart from the traditional zones / instances, and with one of the only two new features collapsing before our eyes not even a month after Blizzcon (warfronts - apparently there is going to be just one and that's their current plan "for the expansion", read one of the recent interviews), I have a hard time seeing what the heck there is even to do for so long. So, maybe they are going to release in something like May, I don't know. A BS empty expansion for the same cool price of $60, but "early" (so, go celebrate, woohoo /sarcasm).