This will actually depend on how long it has been on and how hot it is. There should be heat or thermal sensor and depending on what that is set to is when it will switch the fan on or off. I assume the unit is in a cabinet? IF there is insufficient airspace around it, then heat would be retained longer and therefore the fan would run longer trying to cool the unit down.
You could try a quick experiment, by switching the grill on for a minute then switch off and see if the fan runs for the length of time you have said. If it dos then I would be suspicious of a sensor.
If you have the manual see if they give any details in there or alternatively contact the manufacturers for further info.
Andy A, April 2010
