Don't know when it'll be added. Code and BIP are still in progress, although Pieter will do a presentation on SW at Scaling Bitcoin. My off-the-cuff guess is that there will be a pull request by the end of the year, with at least 2-3 months spent testing it. Won't make it into Core 0.12, not sure about 0.12.X, should be in 0.13 if devs are happy with it and the miners look like they'll play ball.
My 2 cents, of course.
This thread has some more info to tide you over for now.