I don't know. No one knows.
This is Hollywood, friends. As William Goldman wrote in ADVENTURES IN THE SCREEN TRADE, in Hollywood "nobody knows anything."
I do know that David & Dan have stated in interviews that they see the show winding up in seven seasons. People send me emails about that every time I say "I don't know" in response to the question, as if I were somehow unaware of what D&D have said. I'm not.
I also know that HBO wants the series to run longer than that. I have known that since the very beginning... well, actually, since the day after the second episode of season one aired, when I had lunch with one of HBO's top execs, who told me, "We want this to run ten years." I allowed that ten years sounded fine to me. I continue to hear similar sentiments from HBO every time I have meeting with them, be it in LA or New York.
HBO usually renews their shows for one season at a time. It was very unusual, last year, when they stepped up soon after the season four debut to order both season five and season six. Bloggers and journalists can speculate as they please, but none of it will mean anything until next year, when season six begins to air. Around the time that the second episode of season six is showing, HBO will order season seven, we hope... or maybe, just maybe, they will order seasons seven AND eight. But even if they just order season seven, that doesn't mean that the show will end in seven; it could just represent a return to the original policy of committing to only one season at a time.
As we go forward, I expect I will have a voice in all these decisions. But mine will only be one voice among many, and there are all sorts of other factors that can come into play. I will say, I am incredibly fortunate in having partners like HBO, and David Benioff and Dan Weiss. Seven seasons, ten seasons, with or without one or two feature films... in the end, all that matters is that we tell a great story, with a great end.