It is from the time you switch the lights over to 12/12. Some people wait to see preflowers or alternating nodes before switching over from veg, as these are a sign that the plant is mature enough to start flowering. The longer the veg the more yield (hopefully). They can double, and in some cases triple in size in flowering. Best of luck!
i do it different, i start counting flowering time when the sex is determined, I've been on 12/12 for 20 days but i only found out the sex day 17, so i been flowering for 3 days
I think its a personal preference with the times given on beans I order. I found If I harvest at day 65 (begin at 12/12) on a pack of 65-75, I get a lighter buzz. Most of the time, I shoot for the later of the time-frame. If a pack says 30 days, I'd guess about 30-37 from 12/12. just my 2cents