I like to base when to harvest by the overall look of the plant, not just the trichs.
When the orange/brown/amber hairs are sucking back into the bud and the leaves
are yellowing, you know that the plant is reaching the end of it's life cycle. This is
when I harvest.
I am not saying you shouldn't base the harvest on trichs but, in my eye, if the plant
hasn't fully developed than you might be loosing that extra THC or other cannabinoid
development.
Anyways, like monkeybones said it's based on preference and you could chop now if you
wanted. Hope this helps.
-Rotten