I am growing the "heal-all herb", Prunella Vulgaris, and am planning to dry the leaves and flowers in order to make a medicinal tea. How long is it safe to keep the herb once dried? I would prefer to do a large a batch as possible.
For dried herbs to conserve well, it should be dried properly to reduce the water content to below 5%. Afterwards, you store it in airtight jars, to protect it from humidity and sunlight. Especially avoiding humidity is important, dry herbs can easily absorb humidity from the air and this will allow the herbs to start to oxidize and become stale sooner.
If you plan to harvest a lot, I also recommend to portion and store the herbs in separate jars. Keep one jar for daily use, and store the others away in a dark cabinet. The reason is that every time you open and close the jar, some humidity will go in, and some aroma of the herbs will be lost. The storage of herbs is in fact not much different from storing dry tea leaves. Thus, you may have a look at this guide which I published on linked in: Tea Storage for Business. It's maybe too much, but it will be useful for you.
If properly dried and kept airtight, it will keep safely indefinitely. If it spoils it wasn't dried enough. Active drying should of course be started promptly after picking. But safety of dried herbs is rarely the issue. Loss of flavour is more likely to be the limiting factor.
Many herbs lose flavour just from the drying process, others stay tasty for a long time, and some are in between - they can handle drying but not for long. I've seen attempts to categorise herbs to predict how well they'll taste good after drying, but haven't found them convincing. Mint and some related species keep fairly well, but basil (for example) isn't really worth drying.