A lot of fans have been wondering what is going to happen with the HBO series once all of George R.R. Martin's books are used up as material. The author is still writing his next installment of the book series, but HBO isn't concerned with it at the moment and they plan on moving forward with the series with or without the material.
HBO Programming chief Michael Lombardo had this to say to the Television Critics Association:
"We're not off on our own in respect to at least next season. Obviously George is an integral part of the creative team, so next season every move is being choreographed very closely with him. Certainly after next year we'll have to figure it out with George. The book's not finished at this point, but we're in conversations with him. We're not concerned about it."
He also clarified previous movie rumors saying, "there's no conversations going on about a movie." He also said that they are "not talking about ending the show in the next two years." That means we'll get at least three more seasons.
Source: TV Line