Well, all plants are different. Mint you can mow to the ground and as long as the plant is well established, it will bounce back quickly. While pipsissewa (Chimaphila umbellata), which is a medicinal herb, grows slowly and from what I have read does not bounce back well from any harvesting.

In short, you will need to learn more about each plant and do some experiments. If you watch the plant, it will often tell you exactly how much and how often you can harvest. A vigorous, quick, and almost uncontrolled grower, like mint or oregano, can be harvested often. Basically wait until the plant has regrown most of what was cut before unless you were "renovating" the plant. It should look like a plant that hasn't been pruned or harvested when you go back to harvest more.

With a slower grower like garden sage, I pinch out terminal leaf pairs and buds just short of a pair of nice strong leaf buds. In time those buds will grow to each be new stems. It often ends up being much less than a third I harvest at anytime from my sage. And it does take a while for those new leaves to grow and get large and healthy - definitely much longer than for a mint. But the plant gets bushier with each harvest and the yield does increase with time though I never end up taking a 1/3 of the plant. Oh, and as with the mint, it doesn't look like a pruned or harvested plant when I go back for more. I can tell it has been shaped and cared for but it looks lush. Not pathetic and struggling with bare spots and a stick-like structure.

As for young plants....I also never harvest the first year for perennials. Only lightly or to shape the second year, unless they are very vigorous like mints. The third year is when I start to harvest normally. I want to keep the plants over the longhaul and allow them to become established before taking away from them.

FataMorgana