I don't believe there is one way that is correct as with alot of things. I could be wrong there but I think some people count each way. I always start counting when the plants start to show sex not when I change lights. Thats just me, it makes me feel like I am being truer to my plants. But I also don't base me harvest on a specific time frame. I'll use that time frame for referance and keep a close eye on the plants when that time comes. The white widow I've been growing from clone for 4 years gets pulled right around 7 weeks. I've pulled it as early as 6 and as late as 10, and haven't seen a noticable differance letting it go any longer then 7-7.5 . It all comes down to the genetics, and how well the plants are growing, how much stress they've had, and such.
Peace TC