ideally you want to wait until the plants are mature.. you know they are mature when you see alternating node growth and/or preflowers... there are a few other variables that need to be considered tho...
are your strain sativa or indica dominant? sativas stretch more than indicas, so you want to allow more room for them... either way assume you are going to have 2 to 3 times the size of plant coming out of flowering as it was going into flowering...
what kind of lights are you using?? if you dont have sufficient lighting to grow large plants, then dont... a good guide line for penetration ability from a light is..
400w hps = 1 ft
600w hps = 2 ft
1000w hps = 3 ft
those two things should be figured in... if you can wait long enough to mature the plant, the final product with be much better, more potent, and a larger yield... you will still get good smoke from a pre-mature plant, but it wont be 100% of what it could be... if the clones were taken from a mature plant, or if you take clones from a mature plant, they themselves will already be mature and are ready to flower...