I have a strain that flowers in 8 weeks. If I flower from seedling will I be able to harvest in 8 weeks?
No. "flowers in 8 weeks" means the flowering process (or flowering stage) takes 8 weeks. It wont start that process until it's sexually mature - regardless of your lighting schedual. Your plant will continue to grow in Veg stage for up to 5 weeks, until they're mature. In effect, you're just vegging them with a highly reduced amount of light energy during this period.
I heard that even with 12/12 from seed, that the plants will grow for about 2 weeks then begin flowering. Can some one clear this up for me?
I already explained that part.
Thanks -Brizzy out
The reason for doing this, of course, is to not waste any more time than is necessary; because the plants spring into "flower" as soon as they are mature. The "tradeoff" is substancially smaller plants, because they should have been getting at least 50% more light hours during thier "veg period".
A better idea would be "12-12 from Preflower" because the plant isn't mature till then anyway and you could be using your lights 18-6 (or whatever) up to that point.
My comments assume you are talking about an actual "seedling" (from seed) - small clones are a whole different story.