This comes from 420 magazine: "Should I Harvest Early, or Late? What's the Difference?
If you harvest early, the THC content will be higher, therefore it will produce more of a cerebral "head high". When a plant is in the later stages of budding, the THC starts to turn into what's known as CBD. CBD is thought to produce more of a body buzz then THC. The high will be less clear and will sometimes make you feel stupid. So the later you wait to harvest, the more CBD you'll be smoking.
I would recommend experimenting a little, no I don't mean you need to find a white lab coat, just try harvesting at different times and "feel" the results. Try and find a balance between the two. When 50-65% of the hairs turn color, it's usually a good time. The right time to harvest marijuana is up to you.
Don't touch those buds! Touch only the large fan leaves if you want to inspect the buds, as the THC will come off on your fingers and reduce the overall yield if mishandled."