No one really knows this answer for sure...
But if history is any indication ...
NCEES started using IBC 2015 in 2018...
NCEES started using IBC 2012 in 2015...
I'm not saying its a 3 year cycle, and if you look at the ICC website of states with IBC 2018 as being adopted, I think you'll be able to count them all on one hand. That said, I'd speculate that this year 2020, will be the last year of the 2015 cycle, and I wouldn't be shocked if next year we see 2018 picked up.
Just my two cents.