From: Here
For Adults
If the temperature is 160 degrees Fahrenheit, it would take 1 second [to cause a third degree burn];If the temperature is 149 degrees Fahrenheit [common home boiler setting], it would take 2 seconds;
If the temperature is 140 degrees, it would take 5 seconds;
If the temperature is 133 degrees, it would take 16 seconds;
If the temperature is 130 degrees [recommended setting], it would take 35 seconds;
If the temperature is 127 degrees, it would take 1 minute;
If the temperature is 125 degrees, it would take 2 minutes;
If the temperature is 124 degrees, it would take 3 minutes;
If the temperature is 120 degrees, it would take 10 minutes.
[Adult skin thickness of 2.5mm]