I think a lot of it depends on the size of the vehicle and the scent.
My husband has a Kia Spectra - a little 2 door sedan. He has the Fresh Linen scent in his car, has had it in there for well over 6 months and it's still very noticeable. He only has the one on his passenger visor.
I have a Honda Odyssey. I have one on my passenger visor also. I vary my scents and can't stand the one he has. I tend towards, Vanilla, coconut, a few fall scents. Typically they are noticeable for a week maximum unless the van is shut up in extreme heat for an extended period of time (like in an amusement park's parking lot and the sun is directly on my windshield. Then I get a faint whiff again. BUT - if any of the passengers in my vehicle decide to add their own personal scent or there is a noxious gym bag, wet dog or other smelly article riding with us, I can grab the scent portable and clip it on my seat belt and it's still strong until the oil is completely evaporated.
So I'm thinking of getting at least one, maybe two more for my van. The van doesn't typically smell bad, I'd just rather it smell good (and not over powering)