Follow the interval stated by the manufacturer for your specific model, since that is what the validation and warranty position rests on. In practice most laboratories running clinical freezing programmes calibrate annually and verify more often, because the annual figure is an interval, not a substitute for routine checking.
Between calibrations, run a periodic verification against an independent reference probe placed in the chamber, and keep the traces. If the freezer supports a stored programme library, confirm after any firmware or software change that the programmes still run as recorded. Calibration should be documented separately from the service visit.
