Since I now clean my own chimney I don't have an annual "inspection" being done by my chimney service and am on my own. I have a straight, 8" chimney, normal black piping up to the ceiling and then the metalbestos for the section through the ceiling and out the roof. It gets moderate to heavy, but not extreme use for a good five months of the year. About six years of use on it so far. Outside and inside visual inspections leave me to believe that all is well, but I am just what kind of life expectancy is to be typically associated with piping materials. Is there a time frame in which they should be replaced or will they outlast me as long as they receive proper periodic cleaning? Thanks in advance for any insights.