From what I understand, even if you use 12/12 from seed.
Plants HAVE to have a certain time period of vegging before flower.
In other words, 2 week old plants aren't gonna start showing flowers even under 12/12 lighting.
However, you can prolong vegging by not using 12/12 lighting. Your plants genetics determine
the minimum amount of vegging time before they are able to flower.
So in order to get them flowering as soon as your plants genetics allow, set lighting to 12/12
sooner, rather than later. However, the trade-off is that if you only veg for the minimum
amount of time, you will not have as big a yield as if you vegged for at least a month.