How long does one brick last?

Depends on cabin temperature and airflow, but solid bricks outlast hanging fresheners by weeks. When the scent drops to barely noticeable, swap it out for a new one.

Will this stain my cup holder or dashboard?

Solid brick fresheners don't leak or sweat like gels. Just keep it on a clean, dry surface and you're fine.

Can I use this in a hot car parked in the sun?

Heat speeds up evaporation, so scent will release faster in summer. Store it in a shaded spot or remove it on days your car sits in direct sun if you want maximum lifespan.