The times given on the packets are just a rough guide and not an exact formula. Each and every plant will mature at different time frames and are dependent on temps, nutrition, grow techniques, light intensity and mostly the individual plants' genetics.
If you are after a clear, psychoactive, motivating and "up" high, harvest when there are still roughly 5% whitish pistils, or if you prefer a heavy, lethargic and sleepy stone, wait until all the pistils have browned off. Just be aware, late picked flowers have lowered THC and higher levels of CDB and CBN and that is why late harvested flowers are more "couch locking" than earlier picked flowers, which contain more THC and less CBD and CBN.
Hope this helps,.....
Organoman.