Definitely at the point of scoping trichomes and reacting to the level of amber you want to see at various locales.
if you don't have a 30-60 scope to observe trichomes, get one. jewler's loupes / currency checkers are cheap handheld options.
how much amber you want to see to trigger a harvest is up to you. There's a range and you may need to experiement a bit to figure out what you prefer most.
Some plants don't amber-up like others, and in those cases i've accidentally allowed a plant or two to over-ripen. I plan to adjust my strategy a bit for this cases when i recognize it... stick to some 7-14 days period after it has fully plumped up and harveset regardless -- next time i see it, i'll try something new in that range... but this situation shouldn't be a common occurrence, in my experience.