wel you gotta think about this.
indoor or outdoor? outdoor, then whatever you want
indoor is where it gets complicated. say you have 100 inches of usable height. subract maybe 10 for the light system, another 12 for the space between the HID lamps and the canopy (so it doesnt burn), and subract the height of the pot (mine are 24in tall), that leaves you with about 50inches of total plant area. if the plants double or triple in size when switched to flower, then you need to switch them when they are about 20-25 inches tall
also, if you go from 24/0 to 12/12 in 1 day, then they will stretch a LOT.
if you go from 24/0 to 20/4 to 16/8 to 12/12 over a period of 2 weeks the stretch will be a lot less.
at the same time, if you own string, then you can fuck up, switch too late, have them go too tall, then tie them down some.