I'm guessing it'll be a while, like 8 years or so. We're nearly 2 years in, and the devs are still coming to grips with the hardware (I'm saying that based on all the 2015 -> 2016 delays). I think we'll be pretty impressed by what they learn to squeeze out of these things, after 5 years or so, and so people won't be clamoring for better and better so much. We're getting a declining return on investment as we move forward in these generations -- each bump in resolution and whatnot is a little less impressive than the last. Plus, I don't think anyone's going to be in a rush to push the budgets even higher still. So, I don't see any rush into the next gen. I think it'll be a while.
Sony, at any rate, will not be in any rush. I suppose MS could try to get an early start, like they did with the 360. That strategy was very successful for them.
I'm not even counting Nintendo. They kind of just do their own thing. I don't even know what generation they are in anymore.