The National Measurement Institute (NMI) advises that industrial scales should be calibrated every 6-12 months depending on the stresses applied to the scale, or after major hydraulic work is completed on the scale. This is for maintaining accuracy, fairness, and to comply with regulations that could otherwise cost you.

Maintaining accuracy
No matter how robust your industrial scale is, over time it becomes less accurate from natural wear-and-tear. Maintaining accuracy is important for both trade and non-trade purposes, including adhering to the load-bearing capacity of a machine, or ensuring you have enough of certain material for a construction project.
When measuring large quantities of material, even a small discrepancy in the scale’s accuracy can result in large deficits or unnecessary surpluses in material being transported. Maintaining the accuracy of earthmoving machinery’s load bearing calculations is essential to preventing overloading-related accidents, or moving material in inefficiently small amounts. Scale inaccuracy often causes product quality to decrease.
In situations where there is an underlying problem beyond natural wear-and-tear causing scale inaccuracy, putting off scale calibration can cause unscheduled downtime and even product recalls.
Complying with trade measurement law
Trade measurement law requires “an unbroken chain of calibrations to primary measurement standards… to ensure that measurements are comparable to each other and give industry, researchers, regulators[,] and consumers confidence in the accuracy of measurement results.” Basically, scales should be accurate so we know how much of a thing we’re getting each time we get it.
Australia’s trade measurement transactions are estimated to be worth more than $750 billion each year, so maintaining their accuracy has high monetary value.