How long does this air freshener last before it needs replacing?

Longevity depends on cabin size, ventilation, and how much you're driving. Solid bricks gradually evaporate, so you'll notice the scent strength fade over weeks to months of regular use. Replace when the fragrance becomes subtle.

Can I hang this from my rearview mirror, or do I need a vent clip?

Either works. The brick is designed to hang in multiple ways—mirror, vent clip, or loose on your dash. Pick whichever spot gives you the best airflow and doesn't block your view or controls.

Will this stain or damage my interior if it gets wet or bumped loose?

No. Being a solid brick, it won't leak or bleed liquid onto fabric or plastic. If it falls, just pick it up and rehang it—no cleanup needed like you'd have with gels or sprays.