2-4 weeks?
get a 60x magnification.. $5-7 or somethign more expensive like a digital microscope etc... google "trichome harvest marijuana" and you'll have 50 opinions that vary slightly about what is "correct"... do your own testing of an "early, mid, or late" harvest and decide what you like at that point.
in gneeral, the visuals always lag what's actually happening. they look 'ripe' before they are ready. rely on trichomes w/ magnification, not your eye. ignore what the plant looks like, except to gain familiarity with the process, of cours.. only in regards to "when" ignore it..