I waited until my plants showed me their preflowers which i saw with my 30x loupe and once I was sure I saw the teardrop shape with 2 green hairs, I switched to 12/12
For timing purposes, I have found using the first definitive sign of sex as the beginning of the flowering stage much more accurately reflects the recommended time of flower for a strain.
When I see those two beautiful white hairs searching for pollen, that's day one. This grow I vegged my plants long enough they were screaming to be put into 12/12 so the day I switched I counted it as day one. Two more days until my sour cream hits ten full weeks of flowering. She'll go one day into week eleven and then it's curtains for her.