How long are bottled dry-herbs good for?

Title says it all.

I guess I would only try to discern a bottle never opened versus one that was.

The general rule is that immediately after opening they start to slowly lose flavour, but it takes 2 - 3 years after opening until the potency loss is noticeable and you need to add more to bring the flavours level back up to the recipe amount. However, that varies with whether the whole or ground. Ground / powdered herbs don’t last as long.

Oxygen reacts with the herbs (and spices) causing their flavour loss. Ground products have more surface area to react with the oxygen, so they lose flavour faster.

From a practical POV, it doesn’t make any difference to me since I start with the recipe amount and them add more to taste anyway.

Most don’t go “bad” in and of themselves (and potentially result in sickness). The only exception is if you’re in a really humid environment. We’ve lived in the tropics, approx. 90% humidity always, and we had a big issue with opened dried herbs and spices going mouldy in a few months. We kept them in the fridge.

Unopened, it depends on the container. Best is glass or metal, vacuum sealed with a foil top seal. Glass and metal don’t allow oxygen to go through the container wall. A vacuum sucks the oxygen out of the container and a foil top seal also prevents oxygen permeation.

Plastic containers always allow some degree of oxygen infiltration over time. The shelf life with plastic will vary dramatically depending on the thickness of the plastic, whether it has an oxygen barrier liner inside them (plastic soft drink bottles have this) and whether it’s top sealed and with what.

I agree with all of this, although I haven’t measured “time until you can tell the difference” myself, and would guess it varies by spice. But I would add:

Even vacuum sealed spices will lose some nuance and potency over time.
Light and heat accelerate the deterioration of flavor.

In general, it’s not a huge deal and you can just add a little more of the spice. But when my mom gave me some old spice tins that had been sitting for decades, and I sniffed them, and compared their potency to that of the bottles in my spice rack, I threw the old ones away. Yes, you could still tell it was cinnamon. But mine was SOO much punchier.

I’d agree, but it’s variable. Some are more resilient. Some you can compensate for by using a larger amount. Freshly ground spices are much better, not even close. Fresh herbs are not always much better, it depends.

Taste them. I still use some spices from my parents’ wedding gifts 60 years ago. Some are fine, some are fie if ground, some are not, and there’s o pattern I can discern.