GRRM himself has said this about the "will there we more than 7 books issue":
http://insidetv.ew.com/2014/06/03/george-r-r-martin-ice-and-fire-publishing-plan-game-of-thrones/
“My plan is to finish in seven,” Martin says. “But my original plan was to finish in three. I write the stories and they grow. I deal with certain things and sometimes I find myself not at the end of a story. My plan right now is still seven. But first I have to finish Book Six. Get back to me when I’m half-way through Book Seven and then maybe I’ll tell you something more meaningful.”
Martin has been consistent on this subject, giving basically the same answer when we asked if he was “firmly committed” to seven books back in 2011 (“I’m as firm as I am,” Martin said, “until I decide not to be firm”).
He's also said:
That said, given the amount of story remaining in his 4000-page saga — much of which hasn’t yet been published — Martin suspects the
producers’ plan of making seven seasons will not be enough. “I don’t think it will be enough to tell the story we’re telling in the books,” he says.
Which is also what I've been thinking even without knowing the future storylines - there's a shitload of things to be resolved in those remaining books, so 7 seasons could end up being very rushed.
I was the guy that was arguing of few pages back that the series would be at least ten seasons. Now that I see how much of books four and five are being incorporated into the current season I believe that I stand corrected. The only way to complete the series now in seven or eight seasons is for the television series to take its own path (guided by GRRM's broad outline) and the books to continue the way the author writes them.
To paraphrase what Martin has said previously, fans need to realize that this story is not a "choose your own adventure".
I always thought that books 4 and 5 would be covered by season 5 (especially as some of it is being covered this season, this way they make up 1.5 seasons, roughly) because there's no way to make two seasons out of them; one extended season, yes, but not two seasons. It's not that there's no material, it's that there are lots of characters, locations and storylines, but none of them individually have enough material for two seasons - not even Jon, because the arcs would feel weird and suffer if they were split into two seasons.
I also think they will include some TWOW stuff next season for some of the characters; the battle of Meereen, certainly; battle of Winterfell, perhaps; Sansa will have to have some TWOW stuff - especially considering that they've sped up her storyline (and even without it, I already thought she'd have to have some TWOW stuff, since her AFFC chapters are thin on actual events, though rich in internal monologue), I wouldn't be surprised if they add something from TWOW for Arya, and I don't know what they'll do with Bran: probably some TWOW stuff, and maybe they can use him again for weirnet visions to finally get a bit more of the backstory into the show? The only storyline from AFFC I see possibly moved to TWOW is Oldtown, because it could be too many locations and new characters for one season, and it's just set up in AFFC anyway. So Sam may not get to meet the maesters, "Alleras" and "Pate" until season 6 premiere.
Still, I don't think that two more seasons will be enough to wrap up everything, even without hearing the reports of how long the next two books are supposed to be - there's just too much stuff to be resolved. I think eight seasons would be a more realistic and better goal than seven, unless they make extended seasons as The Sopranos, Oz (which practically did two seasons in one year with a break and called them season 4) or some other shows, but they've said it doesn't fit their shooting schedule.
