The actual wire that is the foundation for an RBA coil will last for weeks or months. The cotton on the other hand need to be replaced more often. The cotton used in with an RTA / RDTA needs to be replace either when you want to change the flavor completely, or when the taste/vapor gets worse which is usually once or twice every month. For drippers, RDA, you’d need to replace the cotton a couple of times a week.
If you don’t change the coil/cotton often enough, the cotton will burn. And to filter your lovely, and probably not so cheap e-juices, through burned cotton is just not a good idea.
Check out how a burned coil looks like (mmm...):