Stonerdanswered grow question 4 years ago Best way to know for sure is using a loupe to check the trichomes but if you dont have one then just wait until buds stop fattening up and more than 70% of the pistils orange/brown- this is still a guess and not an accurate way to know!!.
Btw your plant shows severe signs of Potassium (K) and Phosphorous (P) deficiencies which are easy mistakes you can make during flowering, check your pH and make sure its between 5.5-6.5 (6.0-6.5 is preferable for soil) and increase what ever PK/flowering fertilizer you're using by at least 50%.
if all goes well I believe you'll harvest in about 2-3 weeks.
Hope this helps! Good luck and happy growing :)