Does this need to be mounted, or can I just set it down?

Set it down. No adhesive, clips, or fasteners needed—just place it on your dash, console, or cupholder. It'll stay put during normal driving.

How long does the 145g tank last?

That depends on cabin ventilation and how quickly you want the scent to fill your car. In a sealed cabin it'll persist longer; with AC or windows open it fades faster. Expect weeks of noticeable scent under normal daily use.

Can I refill this or is it one-time use?

It's a single-use tank—once the fragrance is gone, you replace the whole unit. The novelty can housing isn't designed to be opened for refills.