So Blizzard just announced the actual release date of BfA (August 14, 2018).
They also just added Legion to BfA for the same price as BfA alone.
Pre-patch will likely hit 1 month before that with PTR testing a month before that. So BfA is essentially going to be done by June. This is perhaps the fastest release of an expansion for Blizzard ever.
Assuming they can keep that date.
I question the depth of this expansion given it's short turn around.