Yeah iOS is pretty much getting saturated. Apple ignored the developing world until now, so Android's got most of those markets. In Europe and America i guess whoever wanted to get a iDevice has got one. Plus i doubt anyone's ever going to use all their iPhones together even if they get one every year. It'll still be one active device.
Same is true for other devices, but then there are more people that don't have iOS stuff than ones that do.
Windows 8 should be pretty cool for mobile devices, and Intel's stepping up its game, so i see a lot of potential in that combo. MS will allow native code apps, so that should allow the gaming market to flourish. Plus MS has its all time secret ingredient, DirectX. I 'm not sure, but i think that'll allow game designers to ignore the graphics side of hardware, i.e. similar to what happens in the PC gaming market.
i put Android's time closer to 3 years, unless Win 8 devices are all more than $300, in which case it's hard to say what'll happen. After all, Android's strength is that it's accessible to almost everyone, in terms of cost.
The WP7 road pretty much ends after Win 8 releases, unless WP7 devices move into the lower-end segment (and assuming that $300 is the threshold for Win8 devices).
/speculation