They don't really know when they will end it, and they are finally acknowledging they may run into a problem running out of books before Martin gets off his duff and publishes.
http://insidetv.ew.com/2013/06/09/game-of-thrones-future/
This statement from Martin in the story really ticks me off:
"Martin points to Starz’
Spartacus, which interrupted its main storyline with a prequel season. As it so happens, Martin has discussed with HBO the possibility of developing a series based on his Hedge Knight books, which are prequels to Ice and Fire."
Someone in the comments enumerated how I feel about this statement. Martin is trying to equate Andy Whitfield's cancer with his laziness and disorganization? That is awful and shame on Martin! It is not even remotely the same circumstance. While I would find a prequel about Robert's Rebellion interesting, Martin had better not be involved in the writing and taking away from writing the ASoIaF books. He just needs to get to work. Since people know that he has already told the producers where he intends to go with the story - in case something happens to him - Martin now has people wishing for his death. This horrible sentiment will only intensify as time goes on without the next book.
I definitely do not want to wish for something this horrible! But I do think he needs to stop adding and just start pulling everything together for the end game. Even with 4 and 5, after he split that into 2 books, he should have been able to publish 4 immediately. Instead he adding more and delayed. Martin needs to stop. I see a lot of complaints online about how characters appear for a few pages, never to be seen again. He should just start saving new characters for some other series. He stated that when trying to publish 4 and 5, they were partially delayed b/c he had, among others, "superfans" reviewing the drafts for continuity. what? When you cannot keep track of your own books, that says to me it is time to stop with his insanity and start editing.
They have already been pulling out of books 4 and 5 for this season (Theon with Ramsey). And they will definitely need to mix books 4 and 5 together for several seasons. This is necessary to keep main characters on screen and to make more sense to tv viewers.
Martin broke up 4 and 5 by character, not chronologically. So you would have certain characters onscreen and then disappear for at least an entire season. That would not work for tv. They need to keep the tv show chronological, so they will be jumping all over, bookwise.
I also think they will need to add stuff to Dany's storyline. I see a lot of complaints that her story really starts to drag and she does not have much to do.
I also think that 7 season statement was made before GoT became so successful. HBO had no clue this would turn into what it has.