Like I said it depends on the strain and the weather. Usually I find the plant is finished when 80-100% of the hairs have turned. However I have found at times that even when 100% have turned the plant still might not be quite ripe yet. Also most plants will start to telescope and push out new, white hairs once they go past being ready. So waiting for 100% of the hairs to turn could mean harvesting too late as many plants still keep growing out new white hairs long after they are read.
Part of it is also getting to know your strains. For example the Mendo Pineapple I like to grow tends to look ready weeks before it really is, but I know that now from growing it several seasons. A lot of the hairs will turn 2-3 weeks before it is finished, BUT the plant itself will still be very green and lush and the trichs won't be ripe, and it will keep packing on weight a resin for several weeks after many of the hairs have turned. That is one strain that I don't really go by the hairs, because I know it's not the best way to tell when it is ready. Another example is that most of the DJ Short strains and crosses, like the Original Blueberry, and the Blueberry pie will not produce amber crystals not matter how long you wait. It is a trait of the strain and something that DJ breeds for. So, on those it's the opposite - I know that looking for amber on the crystals doesn't really show when they are ready so I go more by the hairs and by how the plant itself looks. If most of the hairs have turned, and most of the plant's leaves have turned blue and purple I know it is probably ready.
