Well personally i would wait until i see hundreds of hairs but you can do it whenever you see the white hairs. Reason I wait is because you wont get a lot of seed from just the preflowers if you wait until there throwing out pistols by the hundreds and you will get plenty of seed all the same age. If you do it early and then again some will be more ripe than others and if you leave it to long some might germ right in the bud itself.