I too am growing some Master Kush. They have been 'popped' for about 3 weeks now. Just topped 4 of the 20+ in my grow room. Will undoubtably top the rest when they are ready.
As far as when to throw them under 12/12 for flower, thats kind of set up dependant. How much head room do you have? How much lighting is going to be available?
You can veg as long as you want to, the plants will just keep growing. I always assume that the plants will double in size at least under 12/12, so I plan accordingly. I have an 8ft. x 8 ft. flower room with a height limit of around 6 ft., per plant ( 8 ft. ceiling minus lighting) with (4) 400 watt lamps, in soil (5 gal ). so I usually let my babies go until around 18 inches befor I flip them to 12/12.
I'm not too sure about the caracteristics of Master Kush, so I am skimming through other peoples grow journals to find a little info on the strain, but from what I've read so far, topping, fimming, training, is the way to go to maximize your yeild with them.
good luck on your grow