The major hurdles to new hardware coming out is going to be the development tools. To effectively use another generation of hardware developers will need some quite advanced proceedural generation tools, which will automate creating the high texture graphics and functional levels we expect from a next gen game, but created automatically with some human tweaking.
I expect that will come together quickly once the pieces for it are available, and once that happens all the console manufacturers will leap on it to build a new console. Predicting when we'll hit that spot, though, is really hard.
I expect that Nintendo will completely reinvent itself come 2017 or 2018. Microsoft will quit trying to save the X1 shortly thereafter, and Sony will quit with the PS4 when they know what they're doing after it, whenever that happens to be.
To be honest, if the AAA game market collapses (and with Destiny costing $500 million, that looks to happen quite soon, now) I can see the market going the cheap or indy title route and graphics stabilizing for the better part of a decade. Why push graphics when you won't get your money back on it? Why upgrade the consoles when only a few PC developers needs the extra power. Heck, Crytek was going to complain about power, no matter what you gave them.
It's possible this gen could end as early as 2017, but it could also go on until 2028 if the market collapsed around it. My money is on 2020.
