I am here to the rescue, Jack da wack.

Lets say you are growing a Northern Light. You know that you are gonna wanna harvest after ~60 days or 8.5 weeks. You grow your cute little seedling 15 days from the time it pops out of the soil(not even from seed). Then you flip to 12/12. Do you really expect to harvest ANYTHING in 8.5 weeks? Well you shouldn't. Its going to take at least another 2-3 weeks before it even STARTS showing sex.

As you see in my sig, I have 12 females. I flipped them to 12/12 at day 30 from seed. They were a little young at the time. Not sexually mature. They didn't start showing flower for another 2 weeks. After 3.5 weeks i finally got all the males out. While I have had them in flower for nearly 6weeks, they are just beginning to thicken up, and are at the stage where they have no smell, but buds are fattening up.

Obviously different strains mature faster than others, but you are smoking crack if you think you can veg for 15 days and harvest 8.5 weeks later. Yes, you can flower it at day 15, but don't expect a mature harvest for another 10-12 weeks.

My new magic number is 36 days for veg. I think its minimum.