You are going to get a stretch of a least double it's height when switched to 12/12 . So if it's a foot tall expect the plant to be 2 ft after the flowering stretch. Now there are alot of variables that come into play. Examples temp, humidity, light color, light distance, are they sick, are u properly watering plus feeding, and of course strain. Also it doesn't matter if u have a 40/60 that seed u plant can take more indica than sativa. That depends wildly on pheno of that strain.