Twelve months is a convention, not a requirement. ISO/IEC 17025 and most quality systems expect intervals to be justified, reviewed and adjusted based on evidence — which means historical as-found data, usage intensity and the consequence of an out-of-tolerance condition.
Begin with the manufacturer's recommendation as the initial interval. After two or three cycles, review the as-found results: instruments consistently within a quarter of tolerance are candidates for extension, while any out-of-tolerance event should trigger a shortened interval and an impact assessment of measurements taken since the last calibration.
Document the decision rule rather than the individual decisions. An auditor is looking for a defensible, applied method — not a spreadsheet of arbitrary dates.
