Judging by the hairs is not a 100% sure way of knowing when prime harvest time is.

Go down to your local Radio Shack, actually it's called The Source now, ask for a pocket-microscope, and they'll hook you up with a few choices.

I got the 50-80x zoom scope for only $12. You can get a good close up of the trich heads and know exactly when to harvest.

I harvest when about 25% of the trich heads have turned amber.

There are usually 3 stages in trich production.

Clear heads: Harvest when trich heads are clear and you will lose potency and yeild.

Cloudy heads: When all trich heads have turned cloudy, you can harvest anytime with no loss of potency or yeild.

Amber heads: The THC is at it's peak, and no more potency or yeild will come, and waiting any longer, the THC begins to break down.

Peace.