How long a sachet lasts depends on three things: (a) the volume of lavender used, (b) the quality of the lavender, and (c) the contents of the sachet. In terms of volume, good rules of thumb are the more the lavender the longer it will last, and bigger rooms/spaces require more lavender. In terms of the quality of the lavender, not all lavenders are created equally -- a key component for sachets is whether only the buds are used. When the lavender is fresh, the leaves and stems will emit a scent. When they are dried, however, including stems and leaves will basically just decrease the fragrance (the leaves and stems will smell more like grass when dried). We use only the buds in our sachets. And then the contents: it is hard to tell what has been used in the sachet unless you open it up -- some folks might try to make the sachet look bigger by adding plastic beads or other items that do nothing to add to the scent -- they only take up room, make the sachet look bigger, and the sachet will likely lose its scent quite quickly. A quality sachet is one that only has lavender buds or a combination of buds and other scented items (e.g. lavender and cedar). Of course, if you are interested in a eye pillow such as one that is used after yoga or at the end of the day when you want to give your eyes a moment of relaxation, then it is beneficial to include an ingredient such as flax seed (what we use) to add weight the the pillow.
How do you keep the sachet going? They are meant to be scrunched ... while over time the sachet will lose its scent -- periodically scrunch up the sachet -- you should notice that the lavender scent increases and the sachet is helping to freshen and clean the air around it. And when the scent has faded even after scrunching ... time to get a new sachet. We find our smallest sachets last a long time -- even a year later there is still a good amount of scent from it. Our bigger sachets will last for years. Of course remember that the bigger the room the more the air will use the lavender. And the size of the room can affect the longevity of the sachet.