There is no set time, i.e. 3 weeks or 3 months, when to replace a unit. Each individual is different. Through my years, I have found when the density becomes too thin, the color is too faded to blend in, or the base is too damaged to fit well, it's time to replace. I have usually found that a unit really blends well style wise only after it has been worn a while, and gradual wear and tear isn't noticable by you. That's why I set a time limit, usually 6 months, to replace regardless. The old unit becomes a usable spare if it still looks good. If not, I reorder another immediately, so I have a spare ready.