So assuming one book per year, with 11 coming out in 2009, does it sound right that the big old apocalyptic trilogy begins in 2019?
Assuming no schedule slippages, i.e., the same pace of writing and the publisher being of the same mind year after year, and definitely sticking to the idea of only 20 books before the apocalyptic endcap:
11 (Turn Coat) - 2009
12 - 2010
13 - 2011
14 - 2012
15 - 2013
16 - 2014
17 - 2015
18 - 2016
19 - 2017
20 - 2018
21 (Hell's Bells) - 2019
22 (Stars & Stones) - 2020
23 (Empty Night) - 2021