Damn..wish I would have found this thread sooner. Strange no one suggested when to flip.
It really depends on the strain and how much it stretches. From the comments it sounds like your strain stretches quite a bit. I would have flipped them at 30". I flipped my last batch at 24". they're on 44" centers and..well...I either grew them too big or had them too close together. They're on day 30 of flower and are touching each other almost.
I also would have suggested to hang your lights lower. The whole reason of hanging lights vertically is so that they can hang in between the plants and allow all 360 deg of light coming off the arctube. you want to light the sides of the plants. If you are going to hang lights above your plants like you are you should hang them horizontally with hoods. The way you have them now, hanging vertically above your plants, is only giving you a small percentage of the available light.
Better hurry though because I suspect that soon you won't have room in between your plants for the lights to hang.
Anyway..it's looking good. You're gonna have a mess-o-buds soon. Woohoo!!!
My two cents.