8 years, or however long it takes to get GI running at 1080p 60fps and ray/path tracing running at 720p 30 fps. Otherwise there is literally no point in upgrading and I don't see that happening for at least 8 years. The only reason we're upgrading now is that current consoles CPU's are a little slow and they're RAM starved (moreso the RAM starvation). I think the hardware manufacturers agree with this. It's part of the reason there's such an abundance of RAM now.
No GI and path tracing GPU, no 3D RAM, no SSD built into the console = No new console generation. Mark my words (except on the 3D RAM part, we may not get that.. depends on manufacturing).
Nintendo's a different story. Wii U will last 4 years if its stinking up the industry, 5 years if its doing pretty well, and 6 if it ever explodes like the Wii.
People expecting a 5 year generation are seriously kidding themselves.