I get a pedicure at least every four weeks, and this is about how long the polish lasts. I think the nail salons use specialty products - something before the actual polish, something as a top coat and the like. When I try to do nails at home, the polish doesn't last nearly as long.
That said, I'm pretty sure I've seen threads posted around here talking about products that do this...something called seche vite or along those lines?
I don't polish my fingernails, so I am not sure how long polish is really supposed to last. I just know that when I try to do my toes myself, I'm not very good at it and the polish doesn't last nearly as long as when I get a pedicure done in-salon.
During the long, dark, cold winter - I don't get as many pedicures, and will take the polish off myself at the four-week mark, and leave my toenails unpolished for awhile. I'll also trim them, buff up the edges, buff the surface of the nail, and that's about it. I don't go to the nail salon for pedicures nearly as often during the fall and winter months.
Four weeks is my limit for leaving polish on toenails, and that's if it was applied at a salon. If I do my toenails myself, I'll likely change up the polish in a week, but that's probably because I don't have all the right 'stuff' that they use in salons.