What do you think is the best time to transfer the...

Wait until you have filled the canopy then switch to 12/12. Nothing wrong with a short plant, the important thing is to have a full even canopy; a good, full, even canopy is what's going to give you a nice yield. Don't try to force a schedule on to your plants, it's better to learn to read them as their capacity to take nutrients depends on your ambient temperature and humidity and quantity of light. As a rule of thumb, feed with a strength of 0,7 to 0,8 EC /300-400 PPM once a week, Why? It usually takes 7 to 10 days for a plant to react to a feeding, you can't do too much wrong with that strength in soil. Did the foliage become light green or develop signs of deficiencies? next time use a strength of 1.0 EC/ 500 PPM, Did the foliage show signs of overfeeding? Go easier on the nutes next time, or use pure water. It's easy to compensate for a slight deficiency and it won't impair flowering, but a nutrient burn can seriously hurt your plant.
Finally only start feeding them once you notice your soil is not giving them what they need, a good soil mix can last the whole veg and part of the flower stage. For me 3 gallon/11 liter of soil can feed 5 weeks of veg and 3 weeks of flower with only water.