I think this question is kinda like the "when do you start counting flowering time" question. Its gonna be different for almost everyone, but really its up to the plant
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I would generally consider my seedlings to be vegging once their first true leaves open. My reasoning is that from what I've read the cotyledons are mostly there for food storage purposes not for photosynthesis. Thus once those first serrated leaves form and begin to photosynthesize is would make sense to me to say the plant has begin vegetative growth.
Now the plant grows in veg generally until it has matured, and begins to show preflowers. At this point in nature it should be about summer time and the plant will naturally transition with the sun and begin to flower. In our gardens we do cause this transition by switching the plants to a 12/12 cycle. Once the lighting is switched the plants begin a hormonal change inside as a response to having more darkness, and begin to transition into flower. Depending on the maturity of that plant it might take any were from 4 days to 2+ weeks. If the plant was mature before the switch like we discussed, it should take a much shorter amount of time.
Finally its been 4-5 days(we'll say it was mature) since you switch the lights, and you begin to see pistils forming, and the nodes bunching up a bit, and your plant has began to flower. Take good care of it, and show it lots of love, and in (usually) 8-10 weeks depending on the strain you should have some nice ripe buds
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