I'm honestly not sure what to think with this one, mostly because the idea of thinking about the PS5 launch when we haven't even gotten through our second year with the PS4 yet feels odd. On one hand, I could see the PS5 coming soon due to hardware limitations, but on the other hand, hardware is the exact reason I could see the PS4 lasting six or seven years. R&D is expensive, and the video game world is constantly changing. We can't refer to any "normal" five year console cycle when that is a fairly recent trend in the game world that was smashed by last gen because although upgrades would have certainly helped, they weren't truly necessary. It's like now. Yeah, the PS4 won't be on Titan X levels of graphical capability ever, but it seems premature to for developers say they can't get what they want out of the console when they don't know how to get everything they would want out of it yet. The console hasn't been developed on enough for developers to know how to bleed the system for all its worth. For all we know, Bloodborne could have been 60 FPS, but From Software just didn't have the time to keep tweaking the game to figure out how they could accomplish it while keeping the rest of the game as it was.
In the end, I can see this gen lasting until around 2020 pretty easily. If developers absolutely insist the current hardware isn't powerful enough, maybe we'll see things move along sooner, but I anticipate longer gen cycles rather than shorter ones moving forward.