A hanging car perfume is already built to last 2 to 4 weeks without any effort on your part, but a few small habits can push you toward the longer end of that range, or at least make sure you are not accidentally shortening it.
Keep it out of direct sunlight when possible. Cars parked outside in direct sun for long stretches, especially in summer, expose the fragrance card to more heat and UV than one parked in a garage or shaded spot, and that added heat accelerates how quickly the fragrance oils evaporate. If you have a choice between two parking spots, the shaded one is doing your fragrance a favor along with your interior.
Avoid direct, constant vent airflow. As covered elsewhere, hanging directly in the path of an AC or heat vent creates stronger short-term bursts of scent, but it also burns through the fragrance faster overall. If you want maximum longevity over maximum immediate intensity, keep the card a few inches clear of direct vent airflow.
Crack a window occasionally instead of running AC constantly in mild weather. This is a smaller factor, but constant, high-powered climate control does move more air across the fragrance card than natural airflow, which can modestly speed up diffusion. Not a reason to avoid AC when you need it, just a minor lever if you are trying to stretch a scent as long as possible.
Do not stack multiple scents at once. It might seem like doubling up would just mean more fragrance for longer, but two different scents diffusing simultaneously usually blend into something muddier rather than stronger, and you burn through both cards at once instead of one at a time. If you want variety, rotate scents one at a time rather than layering them.
And the most obvious one: once a scent has genuinely faded past the point of being noticeable, usually toward the end of that 2 to 4 week window, that is the natural point to switch it out rather than trying to extend a card that has already done its job. Trying to stretch a spent fragrance past its natural life usually just means driving around with almost no scent at all for an extra week, waiting on a product that has already given you what it had.