I think it's safe to say that Game of Thrones is HBO's biggest show at the moment so it's only natural that they'd want to keep it going as long as they can. HBO programming president Michael Lombardo had previously said that he'd love to see Game of Thrones last for ten seasons, but during the Television Critics Association’s press tour he seems to have reduced that number.
Seven-seasons-and-out has never been the [internal] conversation, the question is how much beyond seven are we going to do. [Showrunners David Benioff and Dan Weiss are] feel like there’s two more years after six. I would always love for them to change their minds. That’s what we’re looking at right now.
HBO has usually been pretty good about concluding shows before they grow too stale, but I really hope they don't let greed get the better of them and stretch out Thrones longer than it needs to be. A total of eight seasons feels right but even once Game of Thrones concludes its run, there will still be a crazy amount of story left to mine in the world of Westeros so Michael Lombardo was asked about the possibility of a prequel series:
I would be open to anything Dan and David want to do, it really would depend fully on what they want to do. I think you’re right, there’s enormous storytelling to be mined in a prequel. We haven’t had any conversations.
Production of the sixth season of Game of Thrones is currently underway and it should be a very exciting season as David Benioff and Dan Weiss have finally run out of published book material from which to draw. It's uncharted territory from here on out folks!