Question about flowering times

Breeders always state about how long you should flower a strain. For example, 8-10 weeks. When exactly do you start counting the weeks? From the time you switched to 12/12 or the time it first showed sex?

I realize this is just a suggestion about how long the strain takes but I'm just curious how they come up with these figures.

Interesting question. I always assumed it was from the time you switched to 12/12.