I think this idea that they're going to start splitting up the novels into multiple seasons a piece is a bit fanciful.
They've already announced plans to do that for the third book. Books four and five will have to be combined into a two season story, since there's no other realistic way to do them.
That's four years right there. If the sixth book is done in 2-3 years, that's another 2 seasons, thus six before the final novel is needed. I think it's feasible for Martin to keep pace with the series production, so long as he doesn't fall disastrously behind again.