Next year we'll potentially see Red Dead, Anthem, God Of War, Spider-Man, Metro, Days Gone, Death Stranding, Kingdom Hearts 3, and The Last Of Us 2.
That will almost certainly mean there won't be any more big games from at least Rockstar and Naughty Dog this generation after that.
Some of those will probably fall into 2019, so I don't think there's any way they'll release new consoles until Fall 2019 at the earliest. And since that's when I'd like to see a new console, but they never seem to come out soon enough for me, I would expect a new console a year later in Fall 2020.
So, 7 years.