I just recently started building my own coils since I figure it's a lot cheaper than buying premade ones, and I've been replacing my cotton every couple of weeks or so. How often do I actually need to replace the coil itself? It seems like if I keep it fairly clean, I don't really need to? When it starts to be a little bit burnt tasting, I know it's time to replace the cotton, but when should I be actually replacing the coils?