It is dependent on strain, some plants will take a longer period of time to get to the phase of harvest.However estimates and averages around strains harvest dates are available for guidelines (eg. 6-8 weeks). To be completely certain use a 30x jewelers scope to check the trichomes (Milky= more psychoactive), this will ensure that you know when they are ready for harvest.If you don't have a 30x jewelers scope readily available in your area, I suggest using the pistils as indicators (white hairs). When the pistils incline inwards and have turned a amber/red/brown, this indicates the plant is ready for harvest.(although this is not a completely accurate method.) Plants are looking frosty, so great work so far.
Hope this helps.