It is my first grow with photoperiodic plants. I c...

About twice bigger at end of stretch

You are going to get a stretch of a least double it's height when switched to 12/12 . So if it's a foot tall expect the plant to be 2 ft after the flowering stretch. Now there are alot of variables that come into play. Examples temp, humidity, light color, light distance, are they sick, are u properly watering plus feeding, and of course strain. Also it doesn't matter if u have a 40/60 that seed u plant can take more indica than sativa. That depends wildly on pheno of that strain.

First of all, take some pictures without the purple led on. You will expect a fair bit of stretch at first. The colas will stretch towards the light. And after a couple of weeks they will show signs of flower and sex (if you're using regular seeds, you'll need to pick out the males). The next couple of weeks will be showing and filling bud sites. And then filling up, bulking out and finally ripening/maturing. So make sure you have a nutrient plan and guide to follow from the manufacturer. Make sure you check your pH and keep adjusting the light level as the plant grows. Any problems you come across, this site is perfect as you can ask literally hundreds of people and answers will follow! Happy growing 🌱😊