I try to keep them all about 8 inches over the top of the screen for the tops to have a little room for when they start packing the weight on. I train for about 14 days into flower before they stop stretching, but some strains are longer. Somwhere between week 2 and 3 most if not all of the stretching is done so you can let them develope above the screen at that point. Easiest way to tell if they are about done stretching is when their stems get tough, almost wood like, but be careful cause they will snap like wood too.
If you are really tight on space though, why not try super cropping? Essentially its the same concept of growing as scrogging, just without the screen.