How long does one 90g can last?

Longevity depends on cabin size, ventilation, and how often you're driving. Gel formulas release fragrance steadily, so one can typically lasts several weeks to a couple months with regular use.

Can I hang this from my mirror or does it need to sit on the dash?

The rabbit ear design allows for both mounting options. Hanging from your mirror works fine; just ensure it doesn't obstruct your view. Dash placement also works if you prefer it out of sight.

Will this stain or leak on my dashboard?

Gel formulas don't leak like liquid fresheners. That said, if you place it in direct sunlight for extended periods, monitor it—heat can soften gels. Keep it in a shadier spot for best results.