If it takes him six years to write a book, then I don't see how there is any way they can't run out of material for the show at some point. Let's say he gets book six out in 2017 and book seven in 2023, and they begin to do two seasons per book. That takes the show to about 2020, assuming they don't get cancelled and viewers maintain interest through four years of books four and five combined. I plan on enjoying the show for two more seasons or so, but it will not last until the end of the seventh book. I'll have to do some reading in about ten years or so to find out how it all ends.
I understand that the author has explained to ending to the show runners. Funny if he dies before finishing the last book, but the show is cancelled, and these guys have the conclusion in their heads with no way to tell the end of the story.