When you go into thermal shutdown on a welder, that's not the duty cycle, that's exceeding it. If you do that enough times, you'll damage the machine.
Duty cycles for welders are based on 10 minute cycles. A machine with a 20% duty cycle will be capable of welding at the stated amperage for two minutes continuously, then requires 8 minutes to cool down. Again it's a ten minute cycle, you can't run for 4 minutes continuous and idle for 16.
Some machines are more sensitive than others. In general, transformer machines are heartier than inverters. Big machines tougher than small ones. Some welders are overrated, others under. If you really pay attention, you will notice the current change when you reach or exceed the duty cycle.
If you have a small machine and only run 10-20 pounds of wire a year, it may take a long time to ruin the machine exceeding the duty cycle, but it will happen.