It is not up to the calibration laboratory issuing the certificate to determine the interval between two calibrations. In fact, accredited calibration laboratories that have signed the ILAC international agreement are forbidden from doing so. The purpose of the requirement for a calibration certificate is make sure that you are in a position to demonstrate the traceability of the measurements made in your company (organisation or laboratory), and, first and foremost, to perform other internal calibrations, either to apply corrections to measurement results in order to improve the performance of the measuring equipment, or to declare its proficiency by making internal verifications (confirmation).