There were originally supposed to be seven books, and George R.R. Martin has written five with the sixth supposedly being written now. He could easily decide to go to eight books though (I think books four and five were originally supposed to be one book but he wrote too much and split it in two). As for the show, over four seasons they have covered ~ 3 books (Seasons 3 and 4 essentially split the third book in half). It's possible they will split other books over two seasons as well. Seven books could end up being nine or ten seasons.

So I wouldn't be surprised if the show ran for another four years. Game of Thrones is really popular now and who knows if it will be as hot by the end. If they're going to make a GoT pinball, now seems like a perfect time.