When to start Flower Nutrients

I'm getting ready to start my 12 hour cycle to induce flower. Plants are about 10"-14" tall. My question is when do I switch from GROW to FLOWER nutrients?

I'm using Dutch Master Gold which has worked very well in the Veg stage, they've exploded in height the last week, and are tall enough in 3 weeks to start flower. Since it takes about 1 to 2 weeks to begin flowering, do I wait until this happens to switch? Does it matter?

i do a transition period of half grow and half flower then after 2 weeks full flower nutes

Have you repotted recently? If not, you can start givin right away... go slow in the beginning tho to see if they like it.

Sorry, should have mentioned. I'm all hydro using 6" rockwool and a "drip" system, although it's not drip, it's a feed for 60 seconds every 2 hours, 1000ppm PH 5.9.

They're under a 600W Super HPS as of today, was using a 400W Super Blue MH up until now.

The plants you see at the front are my autos that started to flower about a week ago. They're on a separate feed of flower nutes right now, 1050PPM PH 5.6.

I flushed all but the autos today with PH 5.5 H2O, no nutes.

I'm growing 3 x Twilight, 3 x UFO, and 3 x Red Dwarf Autos. Attached a pic from the begining of this month, it's incredible how fast these plants grow....sorry, but it's my first time and all I have to say is wow.