The side of your nutrient bottle usually even has a specific measurement for the transition phase with most brands. I've preached this a few times to newer growers that can't figure out why their plants don't have ,many flowers on them but have been in 12/12 for 3-4. The plants take time to transition from veg to flower, and the amount of time they take even has variables.
Its not even as simple as flowering starts 1 week after 12/12, but thats a pretty safe bet if you are growing from clone. The amount of time it takes a plant to transition is effected by genetics, as well as maturity. If your plants are grown from seed it will generally take 7-10 weeks of veg growth for a plant to genetically mature. Some mature faster, and some slower. If you flower a mature plant it will likely only take about a week to transition to flower, sometimes faster. If you flower an immature plant, it can take 2-4 weeks depending on how immature it is.
I havn't counted days on a plant in years though. I keep track of when I put my batches of plants in to flower, and then I watch the plants as they grow. Once that batch reachs its harvest window I check the trichs and pull the plants when they are ready. You gotta just read the plants. I would never suggest harvesting a plant late or early, but when you harvest should have nothing to do with the number of days and everything to do with the maturity of the plant.
