Following up the Vanity Fair piece with its remark that the producers are thinking of seven or eight seasons, James Hibberd at EW has a somewhat more definitive quote from the David Benioff:
“It feels like this is the midpoint for us,” Benioff says. “If we’re going to go seven seasons, which is the plan, season 4 is right down the middle, the pivot point.”
Of course, it’s not 100%, as HBO executive Michael Lombardo implies a possibility of 8 seasons, which matches Vanity Fair’s earlier quote. But at the moment, 7 seasons appears to be the end goal of the producers.