The second and third novels were produced over a four-year span. Hopefully he can get back to that kind of schedule. As I've said before, given how much money is now at stake for various parties, it wouldn't surprise me if he quietly makes use of a ghost co-writer to speed things up.
I read somewhere, but can't remember where, that Martin once described the series as a trilogy--A Game of Thrones, A Dance with Dragons, The Winds of Winter.
But not as many might think: originally A Game of Thrones ended with the Red Wedding. If that is true it would explain why he was able to get the first three books out so quickly--they were originally one novel that he decided to break into three, sort of like Lord of the Rings but by author rather than publisher fiat.
Of course that might also mean that by his original plan we may still only be in Book Two based on where Crows and Dragons ended.