720 and Wii 2 2011
PS4 2012
In Nintendo's case, they have some pressure to upgrade sooner, even if they have a lot more pressure to stick with the Wii. Internal Nintendo employees, like their artists and such, will want to work on more advanced hardware eventually, and Nintendo could easily do what Sony has done with the PS2 and PS3, continued to promote the PS2 as a low-cost, casual device, but to promote the PS3 as a higher-quality product. I know that strategy can seem counterintuitive, but i'm confident that they'll make the Wii 2 compelling enough to really not worry about the Wii undermining the Wii 2
For Microsoft, they're right where they want to be. Firmly trumping Sony, but can't catch up to Nintendo. They'll cultivate their platform for another 2.5 years or so, and then do a steadier transition from the 360 to the 720. Indeed, there was a rumor that Microsoft might make "Forwards compatible," that you could play, for instance, Gears 3 on the 360 at minimum settings, and then see a much more vibrant, advanced version of Gears 3 on the 720, to build a library for the new console ahead of time.
Sony will want to drag the PS3 out a little longer. The longer the PS3 is out, the more profitable it should become. They'll hold it back a year because the next generation won't be so much of a drastic leap that Sony will feel compelled to "keep up with the Joneses," and will instead try to maximize their PS3 investment. It's not like Microsoft and the original Xbox, where Microsoft was hemorrhaging money even in the Xbox's last days, because they were simply unable to get operating costs down. Sony will want to see some profit first, before taking the plunge again
