Duty cycle is a measure of the work that the transformer is doing. Machines are designed not to exceed 50% duty cycle. All components of the machine cables, shunts, transformer, controls and all conductors are designed for this value. If exceeded the life of the machine will be shortened dramatically. The formula shown below is based upon standard AC - 60 cycle welding with a one minute integration time period.
A complete example of the calculation of duty cycle is shown in the article linked below. This example covers calculations for both AC and MFDC equipment.
WHAT IS TRANSFORMER % DUTY CYCLE?
Reference: RWMA – Resistance Welding Manual Section 19.3