okay this refrigerator has an electronic control board that controls the entire refrigerator.
this control runs everything from the compressor to the fans and defrost. you will need to replace the control. its located in the back of the refrigerator just above the compressor area behind an 8X10" cover. its easy to replace and comes with instructions. its basically a plug and play. (just plugs in) heres a link to order it. http://www.repairclinic.com/PartDetail/Main-Control-Board/WR55X10942/1531075?modelNumber=HSS25GFTABB this is the only thing that controls the time in defrost. hope this helps, thanks Bryan