5-15% amber trichomes are when to harvest. Get at least a 30x-60x scope and check a few spots on the buds themselves. Count 20 trichomes in an area. 1 amber is 5%, 2 is 10% and 3 is 15%.

Earlier and you get an incomplete effect that doesn't last as long. Longer and you lose THC to CBN and get sleepy cannabis, not fun unless you're trying to avoid life.