Switching to 12/12 only STARTS the hormonal changes that bring the plant to flower. It may take weeks for the external sex structure to become visible.
When do human females start puberty, for example? When the first external signs appear, or when the hormones begin to change within the body, often years ahead, as early as 9 years old?
Bottom line, unless you are a chemist and can measure the hormones in your plants, then you have to start counting when the first pistils (hairs) appear. How fast they show depends on the growth habits of the particular strain and the ancestry of the hybrid.
I would use caution waiting to see pre-flowers (the small pistils) because plants will stretch quite a bit depending on the strain and may not show those pre-flowers until they're two or three feet high. Some varieties stretch %100 to %150 of their height so you can see how that could become problematic. If you're trying to cycle or do a perpetual harvest go by a time schedule, otherwise I would start at a specified height (knowing how much your strain will stretch) to try to reach a desired height.
Technically you can start from 12/12 and the plants will basically start flowering as soon as they're ready like autos. The same thing can be done at any point in their life but like AKGramma said it's because it causes a hormone surge, it will take some time before the plant responds. Pre-flowers are a sign those hormones are already present at high levels and so it won't take as long before they ente full flower but as he said that varies a lot with genetics so you still can't really time it.
I have 2 strains in my tent 2x wonder woman and 2x bubblegummer, the WW show'd pistils 4 days into 12x12 and the BG's didnt show for 2 weeks, so if i'm correct in thinking the BG's will need 2 weeks more till they finish as both strains are said to be 8-10wks flower time, I will be checking pistils and tricomes for colour changes. thanks a lot lads