How long does one freshener last?

The 18g gel core typically runs 3–4 weeks of daily driving before scent dims noticeably. Intensity depends on cabin ventilation and how often you're in the car.

Can I hang this or does it have to sit on the dash?

Both work. It's designed to hang from your rearview mirror or sit flat on the dashboard, center console, or anywhere stable. Your preference.

Will this stain or damage my interior?

Gel fresheners are low-risk, but keep it away from leather and cloth if you're concerned about prolonged contact. Plastic and vinyl won't be affected. Don't place it where it can tip or spill during hard braking.