First TIME 24 plant setup. when to flower??

I got 24 plants, 12 deepchunk x strawberry cough and 12 hindu skunk. I have it all on a 3x3 tray. I really dont wanna overgrow in my area. I have about 3 feet between my 600 watt light and the pots. was wondering if any one knows when i should switch to flower. I have been veging now on 18/6 for 3 days and they are about 5-6 inches tall. Any help??? anyones input would be greatly appreciated.

I wait untill they r between 9 & 12 inches. then put them on a 12 hours on 12 hours off light cycle, lowering the light half a foot every few days untill u cant get it any closer without burning them. If u grow them 12 inches before flowering. The stalks should set at about 3 foot 4 weeks later, this is the hight u will get the max yield. and the quickest way too.

yeah i only have exactly three feet between the light and the pot. so i dont wannt them to be exactly three feet and touch the glass on my hood.

Well if u only want them to be short i would cut the top off the plants and put them on flower now. When u put them on 12/12 they still have 4 weeks of growing, till the stalks r set, u'll still get an ok yield