How long does one bottle last?

Duration depends on how often you spray and how big your cabin is. A few quick bursts per week will keep the scent fresh for weeks. Heavy daily use shortens that window — test the spray pattern first to dial in your routine.

Will this stain my upholstery or dash?

Spray in an open area away from fabric and trim until you're confident with the mist pattern. If you do get overspray, wipe it dry immediately with a microfiber towel. Test on a hidden spot first if you're worried about your specific interior material.

Can I use this with the windows up, or will it get overwhelming?

One or two light bursts with windows closed works fine. Start conservative — you can always spray again in 10 minutes if the scent fades. In a sealed cabin, a little goes a long way.