In many cases your QA policies, contract requirements or department procedures will stipulate rules on the frequency of calibration. In other cases, the equipment's operating manual may recommend how often the device should be recalibrated. In practical terms, it is all about risk. The longer the period you leave equipment between calibrations the greater the risk of something going wrong or not being spotted. Because of this, you will find that many standards, such as ISO and ASTM, either recommend or state mandatory frequencies of calibration for compliance. If you are testing to an ASTM or ISO testing standard (eg ASTM E8), it is worth checking to see if it stipulates maintenance and calibration requirements. For example, ASTM E8 requires compliance to the calibration standard ASTM E4 which recommends that test systems and all transducers be verified at least once a year. In addition, ASTM E4 (paragraph 20.3) requires re-verification whenever a test system is relocated.