When they're at the height you want them really. My latest grow (you can check my diary if it helps) I trellised about 10 days after topping, at about 10 inches height. Couldn't see the point in waiting longer and wasting growth time as I want a quicker harvest. I flipped it to 12/12 once I've nearly finished covering the screen. I'll keep tucking under the trellis for the first week of flower until the screen is 100% complete