How long does one 90g can last?

Duration depends on cabin size, ventilation, and how strong you like the scent. Gel formulas generally outlast spray or hanging fresheners by weeks, but exact lifespan isn't specified on the product data — crack it open and see how your nose rates it after a week or two of daily driving.

Can I use this in a hot car, or will it melt?

Gel construction is more heat-stable than liquid, but extreme heat exposure isn't covered in the spec sheet. Keep it out of direct sunlight on the dashboard and you'll be fine. Cup holder or under-seat storage is the safer bet in summer climates.

Does this come with a vent clip or mounting hardware?

The can itself ships solo—no clip included. You can set it in a cup holder, wedge it in the door pocket, or grab an aftermarket vent clip from any parts store if you want it mounted higher up.