Part of me thinks X720 and PS4 are late 2012 releases, at least in one or two markets. With PS3 especially, there is no "Kinect like" shot in the arm for Europe and the USA late in year six-seven on the market - given Sony's PS3 forecast, we're already at the point where they don't expect to see meaningful lifts on price cuts (as far as reaching a new level of hw sales) and so I don't think they can go between systems quite as long as Microsoft (already set to be more than 6 years, probably at least 7 years). Microsoft does have to be careful, they can't just modestly update Kinect / graphics - the machine would be too similar to get many people to upgrade quickly, they'll need another a big leap.
That said, PS3 at $200-$250, and a cheaper Kinect mean that Sony and Microsoft could probably wait until 2013 if they wanted to. Sony went six years from PS2 to PS3, and that was probably a mistake as far as marketshare goes, so I don't know that I'd wait until seven years - PS4 in March 2013 would make a lot of sense to me, with X720 in November 2012 (seven years). Sony is under a fair amount of pressure over the next two years for software as PSP and PS2 completely collapse which is my main reason I think they'll do early 2013 at the latest - PSP / PS2 software is still just enough to keep Sony platforms at 200m games shipped per year right now- but I don't think they're really set up to profit if software drops below that level, and I think it could in 2012 / 2013.
To some degree, I think it also depends on how Wii U does. If it gallops out of the gate, I think X360 & PS3 start to lose some shelf space for games and thats the real issue immediately.