Fire Red and Leaf Green were released in 2004 and since then every remake has come out 5 years after each other.
I think this trend will continue and D/PR will be unveild in 2019 after the first main series Gen 8 title(or maybe they'll throw a curve ball and do a S/M sequel ala Gen 5).
I won't complain if i'm wrong and they comes out sooner ;)