thats an interesting question...what triggers the plant to start blooming in nature is when days start getting shorter...inside most folks flip from what ever in veg to 12/12 and that sudden change makes it start blooming faster than mother nature does... so that being said...its up to you as to what length of light ya wanna start em to bloom...ive wondered and might try an experiment sometime , to see if on a longer veg time in longer light and then flip it to bloom, but still keep it longer than 12/12..say something like if ya had it on 20/4 and flipped to 18/6 it will still start blooming..wondering if the longer bloom light time will make the plant bloom faster or more?  18/6 to 12/12  or like 20/4 to 18/6 or any other numbers as long as bloom light is shorter than the veg light was on