The experiments I've conducted regarding Herbalist-made sachets is that they are constantly expiring wether they are worn or not, but not while they are not in the world. So if you have any, you may as well wear them all the time. It's actually pretty handy, even when not resting. However, a statement from Mark or Poobah, i forget which, in an eariler board, indicates that herbs do not expire when not being worn. Results indicate otherwise though.
Well I would dispute the fact that they don't lose their effectiveness whilst one is not in the World. I was absent for a RL period of about 5 days, and found on my return that my almost new sachets were practically useless. This may be a change of recipe I suppose, but I'm convinced they used to last a lot longer.