Short answer: Ask your auditor.
Long Answer:
While the bottom line is usually “ask your auditor”, it can help to know what to consider when picking a calibration interval. Generally speaking, the amount of time between calibrating an instrument should be decided in a conversation with your auditor. If you are an electrician, this might be your NICEIC contact, or if your are in a wider industry this is probably your ISO9001 auditor. Things to consider when picking calibration frequency are how often the instrument is used, how likely it is to deviate from correct readings, and how critical to the process the instrument is.
How often is the instrument used?
Do you use it every day? Several times a day? Or does it sit in a cupboard until it is used annually? How often you use the gauge should be a major factor in deciding how often to calibrate. The more often an instrument is used, the more likely it is to deviate through use, develop faults, wear etc, and so should be calibrated more often.
How likely is the instrument to deviate?
All gauges will deteriorate over time, but some are more likely to deviate than others. Testers like modern multimeters and multifunction testers are unlikely to deviate with time, and are much more likely to develop faults than simply stray out of tolerance. Similarly, slip gauge sets and high tolerance hand weights, unless damaged, are unlikely to deviate naturally over time. The opposite is something like a torque wrench where, if left wound, the spring can stretch over time, or personal gas monitors, where the cells’ lifespan can make long calibration intervals dangerous.
How critical is the instrument to the process or to safety?
Some instruments will be more important than others in your process. For example, is your torque wrench used to tighten wheel nuts? What would happen if there were a failure? These are important questions to ask when deciding on a calibration frequency. The more critical the instrument is, the more often it should be calibrated.
In summary, an item that is used every day, can degrade over time, and is used on a safety critical process should be checked much more regularly than an item that is used infrequently, is unlikely to deviate, and is used on a non-critical process. Just to emphasise, the calibration frequency you decide should always be agreed in a conversation with your auditor or governing body.