How long does one brick last?

Longevity depends on cabin size, ventilation, and how fresh you want it to smell. With regular use in a car, expect weeks of noticeable scent before it fades. You'll know when it's time to replace it.

Can I hang this or does it need to sit?

Solid bricks work either way — hang it from your mirror, toss it in a cup holder, or place it in a storage area. No special mounting required.

Will this stain or damage my interior?

It's a solid brick, so there's no liquid to leak or stain. Just keep it away from direct sunlight if you want the scent to last longer.