Mezuzahs should be checked by a sofer (certified scribe) twice every seven years to see if they have been affected by adverse weather conditions, or by folding (which can cause cracks in the letters), or if any other defect has occurred. It is preferable to check mezuzahs which are on the outside of a building more often, due to their vulnerability to inclement weather.
In addition, the infiltration of fraudulent mezuzahs in the marketplace present further cause for regular checking to determine the authenticity of the mezuzahs in your possession.
There is an ancient custom practiced by people diligent in mitzvot to have their mezuzahs checked once a year, during the Jewish month of Elul, the month prior to the High Holidays. The Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, would encourage all to have their mezuzahs checked during this auspicious month.
