HI HOW OLD ARE YOUR PLANT WHEN YOU GO OVER TO 12/12 BEST REGARDS HENNING
It depends on the size of your grow space; how much of a hurry you are to get harvested buds; whether you "top" the plants (which takes time to recover and grow out the lateral tips). The plant will double in size when you cut the lights to 12 hours. I go by that. (I imagine them twice the size.).
I usually grow in a 7' tall tent. I don't usually "top." They veg 6-7 weeks. I transplant twice during that time (which sets them back a day or three). Right now I'm
growing a test in a 4' tall tent. They're 3.5 weeks old, just transplanted two days ago. I'm going to top at least one of them. I've been varying the veg nutrients to see what works better (the soil needed much more nitrogen than I'm used to. I've been testing both increased N and overall nutrient strength.). If the veg feeding had hadn't been so hard to pin down, I would have switched to 12/12 without transplanting nor topping (since it's just a test.). But, because I want to continue narrowing down the right strength and NPK ratio, I'm going to top at least one, let them grow more bushy for another week or two.
I've kept them under 5000k "daylight" lightbulbs. No warm light. That helps keep them compact, which can add to the veg time. Their hypothetical "twice the size" is nowhere near exceeding the tent size yet. I think I'm close to a good veg NPK ratio & strength. I doubt I'll reach a point where I have to switch due to their size. But, ultimately that would be the deciding factor. The larger they are, the more yield. I'm not growing for yield (just a test if cactus potting mix will grow cannabis.). I would have been happier switching to 12/12 two days ago (if I hadn't overfed them the prior four feedings, still testing "what does this do?").
So, there's a lot of factors. If your veg light causes plants to have larger node spacing (taller, more open growth), you would switch sooner. I've seen blurple grow-lights that will keep a vegging plant very compact and tight node spacing. You could probably veg for a year with one of those. (Not that I think that would be good. Just an example of the factors.).
(note: you should probably create your own thread if you want to talk about this more. We're hijacking the OP's thread.).