Chem Dawg

I have 2 Chem Dawg plants in veg, there about 17-18" tall right now. My closet is about 4 ft tall. My question is since they are supposedly a sativa mixed strain, are they going to more than double in flower? I was going to switch them at 24" but now i'm hesitant to wait that long incase they decide to stretch like crazy.

Expect them to double in size when swithed to 12/12