Things change over time, in the past the release rate was 4 chapter per year, but chapters where considerably shorter.
Later they changed it to 2 chapters per year. there is no reason they could slow it down to 1 -1.5 chapters per year.
Here is the list of dates for the Live releases. And yes, before the gap from chapter 11 to 12, it was between 3 and 6 months between chapters, and then the gaps between chapters from 12 to 19 have all been between 6 and 8 months. So the 6-month mark is an appropriate time to start expecting chapter 20, meaning it may get added to Beta in about 3 weeks, or it could be another 2 months. Going from 17 to 18 was only 6 months, despite ascended goods and the merchant being added, and 18 to 19 was 7 months, instead of 6, because they told of there were technical issues that delayed it. So I expect 6 months between 19 and 20, and then maybe 7 or 8 months in between 20 and 21, depending on what they still have to create and test for it.
Chapter 19 - Revenge of the Exile - November 25th 2021
Chapter 18 - Team Spirit - April 29th 2021
Chapter 17- Traders of Unur - October 29th 2020
Chapter 16- Embassies - April 7th 2020
Chapter 15- Elvenar - September 25th 2019
Chapter 14 - Constructs - Feb 5th 2019
Chapter 13 - Amuni - August 2nd 2018
Chapter 12 - Elementals - January 18th 2018
Chapter 11 - Halflings - May 24th 2017
Chapter 10 - Sorcerers and Dragons - Feb 9th 2017
Chapter 9 - Wood elves - October 12th 2016
Chapter 8 - Orcs and Goblins - July 6th 2016
Chapter 7 - Fairies - March 3rd 2016
Chapter 6 - Dwarves - October 4th 2015
Oh, and this list does not include the gap between release of the game and the adding of chapter 6 because that is different depending on Beta, US, German, and whichever other servers were launched with only the original 5 chapters.