People usually go by personal taste and what they like in their home. Some prefer fresh citrus scents, others heavier ones like vanilla. Sometimes you try a few before you find the right one.
Yes, some people use them in cars too. It's just important that they're not too loud, because in a confined space it can become intrusive.
People usually say that air fresheners last about a month, but it depends on how often you use them. If it's sitting near a heat source, it can run out faster.
Sometimes it's a matter of placement - if they are placed in a place with good air circulation, the scent spreads better. It also depends on the size of the room.
Some people are sensitive to certain smells, so it's a good idea to try something milder at first. If anyone in the house has allergies, be careful with your choice.