What causes words repeated in your head to start fading away from consciousness?
If you use a slightly different (and to my own experience more common) phrasing of this phenomenon to look for information you find immediate relevant results on Google. Rather than referring to words fading from consciousness I've usually heard it being stated as repeating a word often makes it lose its meaning.

The first hit is a Wikipedia article on semantic satiation.


  Semantic satiation (also semantic saturation) is a psychological
  phenomenon in which repetition causes a word or phrase to temporarily
  lose meaning for the listener, who then perceives the speech as
  repeated meaningless sounds.