Cotton was fine for flavor, but the life of a Rayon wick is what made me continue with Rayon. For me, Cotton was twice a week wick changes. Rayon, a week to 10 days. I really think I could go longer on Rayon with the juices I vape.
After a week the outer coil surface is crusted over enough to affect flavor, but the wick ends are clean and the wick center only has a grayish color where it was under the coil. I change it only because I need to do a light dry burn to clean the coil.