i always like to wait to flower till i see preflowers, but that's just me, and plenty of people flower from seed, what is called 12 / 12 from seed..
you can always try some lst of super cropping on them if they get too tall.. lst is simply tying them down with some string or w/e so halt vertical growth, and is known as low stress training as it doesn't really stress a plant out the way that say supercropping is said to do..
with supercropping, you take the stem between your thumb and forefinger and squeeze it so that the inner wall of the stem breaks and the stem is no longer able to support itself.. it will now want to lay at like a 90 degree angle, and after you do it for the first time, you'll ask yourself wtf did i just do to me plants, lol, but don't fret, they will be just fine..
you can look on youtube for videos on both supercropping and lst if you're interested in seeing how they work, but if not, i wouldn't let them get too much taller before inducing flowering just to be on the safe side if you don't want to train them at all..