My neuro, who does a wartload of research on AVonex and Tysabri, told me that Avonex "hits its full effect around 6 months," and when I was having symptoms at that time (actaully for me it was 7 to 8 months) did not feel that Avonex had been given a full chance.
I think 6 months is jumping the gun a little, just based on the words of one of the major researchers, but I can't tell you that I really know.