Anyway it is better to replace the case every 3 months to reduce the risk of getting cotaminatated as the doctor suggest . It is the same thing as we should replace our toothbrush or towels every 3 month . But most people don't aware that . For me , I will not replace my case unless someday I suddenly aware that I should have a new case . one of my friends keep her case for almost 10 years . unbelievable ,right ?