Well, I'd expect at *least* a year, as the most powerful phones out there would currently be brought to their knees by the OS. My current hope for my next phone is something with Windows RT that works as well as my Surface RT and has an HDMI output port. All of the sudden, my phone isn't just a PDA/stopgap measure, but a very functional work "computer."
As it stands, for many types of productivity, Windows RT blows Android - which I used for note-taking/paper writing/research for almost two years - and iOS out of the water. The more they refine the OS and the more form factors they get it into at reasonable prices, the happier I am. If they do take too long though, back to Android I go.