When I first see the sign of pistols is the 1st day I count as flowering. Look for 2 white hairs growing out of a calyx by the main stem and the internode of the top leaf, usually about 2 weeks after you put it in flower. Hope this helps.
Do you mean " when do I start flowering " I start flowering at 12" - 14" the plant will double maybe even triple in size. You will see white hairs then buds will form. I'm not sure what your asking but I hope this helps.