Believe the rumors, plants will more or less double or triple in height after you switch to 12/12. How many plants are you planning on growing under the 600 watter? Have ventilation and heat issues been addressed, as far as I'm concerned this is one of the most important aspects of growing indoors next to all the other shit, but still important. More than not on this site people have trouble with there plants getting to tall during the stretch and lose out on light penetration. Have you heard of LST, its a good method to use to keep your canopy even also sgrogs work like the shit. If you can keep an even canopy you have the opportunity to really dial in the light or utilize the shit out of that 600. Good Luck, and let us know whats up so we can try to help. Oh yeah, mine were around six inches when 12/12 went on and the mother fos are between 14 inches and 30 inches. Nice uneven as shit canopy. Thanks