How often should the fan come on in a frost free refrigerator ?
The fan in a frost free refrigerator is supposed to come on throughout. This is so that it can help circulate air all over the refrigerator. Check out http://www.ehow.com/how_6747390_troubleshoot-frost-refrigerators.html for more details.
Depends on how often the door is opened, and how old the fridge is, etc