How long does one can last?

That depends on cabin size, ventilation, and how open you keep it. Most drivers report 3–4 weeks of noticeable scent from an 80g gel can before it fades. You can extend longevity by cracking a vent instead of running full climate control.

Can I hang this from my mirror or does it need to sit somewhere?

The round can design works both ways — place it in a cup holder, on your dash, or hang it from your rearview mirror. Choose placement based on where you want scent concentration and how much you want the fragrance to diffuse through your cabin.

Will this stain or leave residue on my interior?

Gel cans are solid when sealed and don't leak under normal conditions. If you hang it, make sure it's secure so it won't shift during hard cornering. If placed on a surface, wipe the bottom of the can occasionally to prevent any gel residue buildup.