Harvest when 70-90% of hairs have darkened for highest levels of THC.
Harvest when 90-100% of hairs have darkened for a more calming,
anti-anxiety effect as some THC turns to the more relaxing CBN.
It depends on your tastes, it's subjective. You could try cutting half the plant now and the other half later and see which one you like more. That way, next time you'll know what your preferences are.