Using the Legion model we can see patch 7.1.5 launched in early January (January 10th) but this was over two months after patch 7.1 launched (October 25th). 7.1.5 went on the PTR on November 15th which means there was nearly two months of testing. Comparing it to the 7.2.5 patch of Legion we see that this also required two months of testing to get out the door meaning the Christmas holidays had no impact on 7.1.5's delivery.
When Battle for Dazar'alor launches, Blizzard sets themselves an internal clock. They have to have Eternal Palace ready for launch between four to six months after BDA opens with five being optimum and six REALLY pushing their luck. And 8.2 is going to be a doozy with a mega dungeon, new raid and two outdoor areas (one of which is large and somewhat experimental) to test not to mention the new Azerite system. It's also probably the save the game patch. They will need every second of development time they can spare for 8.2.
I think for their own sake, 8.1.5 has to go on the PTR essentially immediately in the New Year. I think it will be deployed towards the end of next week or early the week after, targeting an early March release date (unlocking the two new Allied races and Crucible of the Storms). 8.2 will go on the PTR as soon as they can get 8.1.5 out the door, launch in early May and then the Eternal Palace will open in early July.
8.2.5 will arrive around the same time allowing them to segue into developing the 8.3 series of patches and the expansion finale taking place this time next year.