How long does one can last?

Lifespan depends on heat and ventilation in your cabin. In typical conditions, the gel slowly evaporates and will last several weeks to a couple of months. You'll know it's time to replace when the scent fades noticeably.

Where should I place this in my car?

Cup holders and door pockets are the most common spots. You can also wedge it between seats or under the dash. Just keep it away from direct sunlight and extreme heat, which will speed up gel evaporation.

Will this stain or stick to surfaces?

The gel can leak if tipped over or exposed to sustained high heat. Place it upright on a hard surface or cup holder, not directly against fabric or leather. If contact happens, wipe immediately with a damp cloth.