How long a coil lasts depends on many factors. How much are you vaping, what kind of juice are you using, are you vaping at low or high wattages, and whatnot. If a coil is still giving you a vape you're happy with avoiding any dry hits or other misbehavior, there's no reason to swap it out. If you're getting poor performance and/or a falloff in taste, it's time to change the coil. There is no firm "Change once a week or else!" rule that I'm aware of.