There is no minimum time. Get a loupe of 60x or better and check your trichome color. Clear is immature, cloudy is mature, amber is stale (couch lock narcosis). When you harvest at different ratios of one color to the next is where you can alter the high somewhat, beyond it's genetics (sativa vs indica).
dude what strain is it? how long have you been flowering it for? like mentioned before at least 8 weeks or 60 days in the 12/12 cycle...if this is your first time you will be tempted to pick earlier than you probably should,wait or get a digital microscope off amazon(like 35 bux) and then look for the cloudy/milky thc glands.....you really dont want to pick early...
if you have no microscope you can go by hairs(old school)when like 60-70 % of the hairs change..usually by then they will be frosty with naked eye...stinky and sometimes it looks like glass coating the whole plant(if you done good)...by looking at yours you got a way to go yet...oh yes they also get really heavy at the end...like overnight they will need support to hold themselves up...get string or a net...(you will see stuff on here ideas to help your plants out...)this is caused by all the extra thc etc forming on the nugs....anyway if you bought the seeds online they usually tell you how many days approx to harvest...remember be patient and then enjoy!!!
thats funny cause i saw the one pic ..thought...mmm.looks like white widow..and it is!!!!...funny i am at end of a ww...and kush grow...but i am growing purple kush...
you will be amazed with all the white hairs and it produces a good yield per plant.(widow)..well if you need help we are here....if you wanna see a few of what your in for check my pics...i will post a few more recent pics...mine are due on the 8th of jan....i am flushing them out today...toke it easy!!! otm