With season 6 looming, it would be good to get a feel on how much longer the series is planning to go on for, for the producers and writers' sake if nothing else. Game of Thrones will take about a season to finish properly I'd imagine, and I'd hate for a fairly slow-burning series to get rushed at the end if HBO pull the plug. A notice period of a couple of seasons would help a lot, presuming it's going to finish long before the final book is out (or indeed, the next one). It could go on for years yet, but I doubt it will as viewer fatigue will begin to set in after a while.
Mind you Lost had that luxury, and look what they did with that!