judge by the day you flip to 12/12 then count it as a you would, and harvest when they are ready and next time you know how long it takes from flip to chop for that strain. the only way to fine tune that is with experiance. things i like to do now to be sure my plants are ready to explode into flower the day i flip them is to make sure they have been on a 18/6 light for a while (sometimes i veg 24/0 even tho its not ideal imo) and ive been starting to test this other way of putting them into flower other than just slamming them into 12/12. in nature plants dont have a dramatic shift into flower all the sudden, so the past 2 runs iv eased my plants into flower by going from 18/6 to 17/7, then 16/8 and then 12/12 over the course of a week or so. its pretty easy to mess it up if you have strains in there that want to flower sooner than others. some of mine started wanting to flower at 17/7 but went back to veg after a week. what this does for me other than waste time(caugh) is that it really kicks the plant into gear. i imagine the plants like little kids being held back from the play ground. they see an amazing toy by they are held back and so they become more excited and eager to run for the thing they saw, then when they are made to wait even more while they can still the amazing playset they are finally let go and they burst into play and have so much fun. thats how i imagine they plants are responding to being "teased" with flower.
i do not think that i put my plants into flower early when i do this, because i make sure they are still in veg before i continue. the plant may start to try to flower but if the light is not continusly reduced and its also above 16/8 most plants shouldent flower and will go back into veg. outside plants do a lot more preflowering and all that than indoors where we use a much longer day cycle to make up for less intense light.
my plants are currently thriving and im seeing buds at the end of week one that are about the size i used to see at week 3 so its great results for me, and i no longer have any issues wondering when for sure to start counting those precious flowering days.