Jelly Bean (API level 16-18) was supported for about 2,5 years: It was released in June 2012 and Google stopped providing updates in January 2015.
KitKat was released on 31st October, 2013. If we assume a life cycle of 2,5 years, support should have stopped by 31st April, 2016, which it didn’t.
It’s really hard to predict anything here so it’s even more important to push the alternative projects like Cyanogen Mod, Sailfish OS and Ubuntu Touch.