I can seeing it being a while say 2013 or so - I think they will wait till there is a leap in TV quality - say 4K or a new 3D that is yet to be standarized, and higher frame rates. I hope the next MS Console is seriously uber powerful, like the jump from Halo: CE to the graphics in Halo: Reach. I can see the next consoles including multiple monitor support and/or Quad HD resolution, a solid state hard drive, a massive set of graphic co processors and 3D.
I don't think their will be much lead time, as it kills your current sales if you know the 'new thing' is coming soon. Like the suprise with the launch of the 360S, which was 'here it is today.' I think we will hear rumors/expect it, but I can see it being rather quick from launch to sales.