The short answer is: typically for one year, after which you need to get your unit re-certified and recalibrated, if needed (we do provide this service). Depending on your application and requirements, you may choose to have your unit checked and re-certified by us more frequently. The time between re-certifications is called the calibration interval.
Keep in mind that this calibration interval, the period for which your NIST certificate is valid, does not start with the date on the NIST certificate. It starts on the date the unit is placed into service. Our thermometers typically do not undergo changes in accuracy between the time of calibration and the time they are placed into service (i.e. during shipment and storage).
Also, please note that although the measurement results provided on the NIST certificate can be considered to be traceable to NIST reference standards at the time the measurements were performed, we cannot certify that those measurement results are valid after the instrument has been returned to you and placed into service. You or the responsible party in your organization must have an appropriate internal measurement assurance program in place to assure the continued validity of these measurement results. This program should also specify a calibration interval appropriate for your application. The recommended calibration interval for our thermometers for general industrial and laboratory use is one year.
Should you discover, through your internal measurement assurance program, that your thermometer seems to give inaccurate readings, please contact us to get your unit recalibrated!