How long does one brick actually last?

Gel bricks typically outlast hanging fresheners by 3–4 weeks, depending on cabin temperature and ventilation. Hotter climates speed evaporation slightly. Replace when scent fades to barely noticeable.

Can I use this on my dash, or does it need to sit in a cup holder?

Gel bricks work anywhere — cup holders, under seats, dash trays, even clipped to vents if you rig it. Just make sure it won't roll around under hard braking. Some guys stick them in a small mesh bag to keep things tidy.

Will this stain upholstery or leave residue if it tips over?

Gel formula won't stain fabric like liquid fresheners can. If it tips, wipe up any residue with a damp cloth. No permanent damage, but don't leave it sitting directly on light-colored trim for extended periods.