It should be for rather long time.
If we decide to use some features of android 5 or 6 etc then that will not mean devices with android 4.4 will stopped being supported if will just mean that some features might not be available on them.
On our list of recommended android hardware you will also find devices running on android 4.4.4.
It's also quite possible that at least some of them will receive android 5/6 firmware in the future.
I'd say it's nothing to worry about.