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Calibration intervals should be set from evidence, not habit. If the observed in-tolerance rate at the current interval falls short of target, the interval is scaled by ln(target)/ln(observed) so that the reliability target is met; adjustment is capped at halving or doubling to avoid over-reacting to small samples.
Interval adjustment
New interval = current interval x ln(target in-tolerance rate) / ln(observed in-tolerance rate)
Calibration intervals for instruments used in regulated, safety-critical or accredited testing must comply with the applicable standard and accreditation requirements.
At least 8-10 for the same instrument type and usage class. Adjusting on two or three results chases noise.
No. Accreditation schemes usually cap extensions and require periodic review, and manufacturer recommendations and regulatory requirements may set a hard maximum.