So does the 12/12 flowering time start the day you switch? say I am running an 8 week flowering plant does that 8 weeks start when I sitch or is there a transition period the plant has to go through?
I would give it a week before counting it 12/12. For me that is usually when I see my plants transition into flowering (showing their sex). It seems lots of newer growers harvest too early, it's something I have done plenty! I also used to count flowering weeks from when I flipped to 12/12. I also used the breeders recommended harvest time, which was sometimes too early in my grows. Now I count flowering start after 1 week of 12/12, look at the plants when the breeder says it should be done, and pair those with other indicators of when the plant is done. Swollen calyxes, trichomes, overall look of the plant (size, when did I flush, color of the fan leaves). I think you'll be ok either way, just remember patience is key. An extra week in flowering can make a huge difference in weight once dried and potency.
When breeders state 8 weeks it means it takes 8 weeks of 12/12 for the earliest practical harvest time. Peak harvest time will be a week or two longer. Stress and poor health will slow the process down so may even take longer. harvest when ripe and use the estimated flowering time as a rough guide
Don't follow the amount of weeks a breeder estimates on their website. A lot of people cut their yield short because they follow it by the book. I've heard of 8 week strains finishing in 10 or 11 weeks. If you don't have a jeweller's loupe to see the trichomes, take a look at the amber hairs for an idea of when to harvest.