Music in trailers is better then dead silence. A movie trailer about movie trailers:http://www.youtube.com/watch?v=MbGNcoB2Y4I
10-20 seconds or less or about 30% of the song. Depends on which is less. That is what my teacher told us.
Depends on your meaning of free. You can always edit out parts of songs.
search for royalty free music
less then 30 seconds or a citation of the song.