How long does this air freshener last before I need to replace it?

Longevity depends on cabin size, ventilation, and how often you drive. Most solid air fresheners deliver noticeable scent for several weeks to a couple months. Once the smell fades, swap in a fresh one.

Will this stain or damage my carpet if it sits on the floor?

Solid polyurethane won't leak or seep, so there's no liquid to stain carpet or upholstery. If it contacts bare metal or plastic, wipe it down occasionally to prevent any buildup from collecting dust.

Can I place this anywhere under the seat, or does it need a specific location?

Place it in any dry spot under the seat where it won't roll around or interfere with seat tracks. Avoid areas where it might contact moving parts during seat adjustment.