Q. My dentist said that white fillings don’t last in the back of the mouth. Is this true? A. Statistically, your dentist is correct. According to expert dental researcher Dr. Gordon Christensen, the average posterior white filling lasts seven years. This is not a long time in the mouth. However, when white fillings are placed correctly using meticulous attention to detail and a rubber dam, white fillings, correctly called resin composite, will last just as long as silver mercury fillings or 20+ years.