LFR fully releases in December. Expect 7.3.5 on the PTR within the same month.
7.3.5 release = Janurary
8.0 Alpha: February
8.0 Beta: May
8.0: Prepatch August-September
8.0: Release October-December
Recent expansions have dropped on the same or at least 2 months before the month, a year after they were announced.
Legion August 2015/August 2016
Shitstorm: November 2013/November 2014
MoP: Nov 2011/Sept 2012
Cata: Nov 2009/Dec 2010
The book was pushed to June of 2018. Ik it has no effect on development time but it can indicate something involving the release "season". I expect a fall release since they said before the end of 2018 so anywhere before December 21st is plausible.