I would say at least 2 years (probably 3) till the next consoles come out. If we were expecting them any sooner we would already see the marketing hype starting. I think I read an article about the xbox360 everyday for a year before we even saw the first demonstration model.
Also, I doubt that the next generation will be as powerful as the last generation was in relation to the PC's of the time. What I mean is that while the xbox360 was about as powerful as a high-average gaming PC, the next generation will be more along the lines of a below-average gaming PC. The reason for this is that the console only needs to push 1080p and the midrange video cards of this generation can already do that, by the time the next generation of consoles come out that will be achievable with low end video cards. While I am sure they will all come with blu-ray drives, I think the real emphasis will be on downloadable content.
The big toss up is going to be 3D gaming. If that gets some traction in the next year or so the console makers will make a push for 3D and that will require a beefing up of specs, although I'm sure that even the low to mid-range videocards will be able to handle that at 1080p by then. If it is the main thrust of the next generation, which it looks like it will, then personally I will be out of the market for consoles. I dont do well with 3D.