Curiously, I have found the bow to be a different animal altogether. Not all 'rehairs' are of equal quality. I finally found a guy in Chicago that can rehair the bow in half an hour and charges $50. I love the way my violin sounds after he has done his rehair. I have had others do it and been really unhappy.
But how often should it be rehaired? Don't know. How can you tell? Not sure. I did read that you should rosin the bow every other day and that will keep the hair from wearing so rapidly.
Any knowledge or ideas here?