IIRC it is when they wrap filming. I assume that a composer will read the script (which I believe that Arnold has stated he was going to or already has) which generates his ideas for the music that he or she will write. Then when they start getting footage they compose the score. It's not unusual for a composer not to be confirmed this "late".
"Man of Steel" still doesn't have a composer yet and principal photography wraps in early Jan sometime.