Duty cycle is the proportion of time during which a component, device, or system is operated. The duty cycle can be expressed as a ratio or as a percentage. Suppose a disk drive operates for 1 second, then is shut off for 99 seconds, then is run for 1 second again, and so on. The drive runs for one out of 100 seconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent.
The more a circuit, machine or component is used, the sooner it will wear out. Therefore, the higher the duty cycle, the shorter the useful life, all other things being equal. If the above-mentioned disk drive has a life expectancy of 1,000,000 hours based on a 1 percent duty cycle, that same device's expectancy would probably be about 500,000 hours based on a duty cycle of 2 percent, and 2,000,000 hours based on a duty cycle of 0.5 percent.