I have plants that were put to 12x12 flowering four weeks ago. Each was put in at approximately 4, 6, and 8 weeks old. Each plant is a different strain and had various amounts of training to produce (coincidentally) 4, 6, and 8 main colas. The 8 week old began flowering within a week. 6 weeks, a few days later and 4 weeks, a day or two more. Each would be a different height without training. It looks like the 8 week old plant is going to harvest in a week or so. The others are probably another week or two later. Between the plants (strains) the colas are pretty close to the same size.
I’m going to go by plant size and development of colas before flowering since I can’t rely on genetics. I've read, and shown, a plant only needs 4ish weeks to go 12x12.