How should I store the powder, and how long does a 75 g jar typically last?

Tooth powder is hygroscopic — it absorbs moisture from the air, which can cause clumping and degrade the essential oils responsible for the lemon-peppermint aroma. Store the jar in a cupboard, not on the open bathroom counter near the shower. Always close the lid firmly after each use, and avoid dipping a wet brush directly into the jar; instead, tap a small amount into the lid or a clean dish, then load the brush from there. This prevents water contamination of the main supply. A 75 g jar provides roughly 150–200 brush-loads depending on how heavily you coat the bristles. At two to three uses per week, expect it to last four to six months. Daily use will deplete it in six to eight weeks. If the powder develops hard clumps that don't break up easily, or the scent fades noticeably, it's time to replace it. The product does not contain preservatives designed for long-term water exposure, so keeping it dry is the single most important factor for shelf life.