Depending on strain and speed of growth, in about 2 weeks they should be transplanted. I've noticed that it's time to transplant when I have 3-4 internodes.
Yup what they said + when I grow, I take it personal and attempt to feel my plants and grow with them.......sooooooo I dont put a time or height tag on them but rather I kind of know when its time. But for shits and giggles, about 2 weeks