Answer: By now you have probably harvested and enjoyed many herb snippings, and I apologize for the delay in answering your question.
Basil and oregano can be harvested a leaf or a branch at a time for fresh use or can be allowed to reach its mature size then trimmed back by about half, allowed to regrow and trimmed again and so on. The latter works well for drying (or freezing) in quantity. I usually begin harvesting the odd leaf here and there when the plant has grown about six or eight leaves. Pinching helps the plants branch and grow sturdier so it is fine to start early. I use the pinchings, too. Pinching can be done as soon as the plant is several inches tall and repeated occasionally.
Parsley grows from the center so you can cut off stems from the perimeter as soon as they seem large enough to use. When the plant is very small, take no more than half of it at a time.
I hope these rules of thumb are useful.
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