it depends on alot of variables you dont have listed here, such as age of the plant, whether or not the plants were started indoors or out, etc etc etc.
plants i have started in the past under a 24/0 veg cycle flowered immediately when exposed to any significant period of darkness.
plants started outdoors (as seeds during spring planting) performed as one would expect and began flowering 2/3 through the season.
what i dont understand, is why would you want to accelerate the flowering period if your outdoors? personally, i would want to maximize the veg time under the sun, more veg = more bud in the end.
but, we all have our reasons. if i was you, i would be very carefull about force flowering an outdoor crop, as it is very easy to stress the plant and piss it off... you dont want that, because then it will turn hermi on you and infect your crop with bastard hermi pollen, and, potentially infest anyone ELSEs crop for miles around.
some things are best left to there own devices... like mother nature. if you were indoors, its as simple as adjusting the timers... but outdoors, its a little more complicated... cannabis is extremely photosensitive, and the slightest light levels of natural light 'spoof' the plant into thinking its broad sunlight... thats right, the plant can tell the difference between night and day, but only that there is a difference...