Historically major revisions were released (out of beta):
DO12: Sep 5, 2016
DO11: Mar 3, 2014
DO10: Apr 30, 2011
DO9: Apr 28, 2007
I can't speculate when DO13 could be released, but before being available for purchase, new releases are announced as a public beta for free testing until released. For example DO12 public beta started sometime around Mar 31, 2016 and went out of beta (available to purchase) Sep 5 the same year.
So far nothing has been announced.
I know this doesn't help you much, but GPSoft traditionally doesn't comment on these kind of topics, so that's the best info we have at the moment :)