The yahoogroups are abuzz with whether or not herbs have to be replaced within six months or become useless to the magician.
The simple answer is no.
The long answer is something like this:
Forget all the nonsense of "energy" or "vibrations" you hear new-agers talk bout. That is crap.
The truth is that it is the "idea" of the plant that is the active principal.
From the divine Nous, the idea of the herb descends through the Soul of the World where it picks up its many attributes. In the physical world it is these attributes that are physically represented by its scent, shape, or what have you. Now, when you combine herbs in a mixture for a particular purpose, you focus on one particular attribute that is shared amongst the herbs to make it sympathetic for your purpose. It doesn't matter if the herbs are new, old, wet or dry.
The only time age is of importance is if the herbs are being used medically. And only then it depends on whether the medicinal alkaloids are fragile and known to break down after a period of time.
So no. You do not have to replace all your herbs every six months.