For use in every day cooking, just harvest herbs as you need them. Even the leaves of herbs that have barely germinated are aromatic. But if you want to harvest herbs for storage (dried, frozen, in oil, in vinegar, etc.), be aware that there is a specific time for doing so.
That’s because the taste of a herb is at its most concentrated just before it flowers. So when you see flower buds appear in your basil, coriander, chervil or thyme, that is the very best time to harvest them.
So much for herbs grown for their foliage. If you grow herbs for their seeds (often the case with dill, fennel, coriander, caraway, etc.), do so when the seed capsule starts to turn brown, but before it fully opens.