Well, I say it depends on each individual actually. Someone who bows with a vengence will wear out the bow faster than someone who's always trying to mimic whisperings. Someone who carefully moves the bow from the pegs to the playing position would do better than one who removes it with a flourish, pulling out a few loose strands of hair in the process.
But here are a few tell tale signs that your bow is due for a change:
1) The Erhu bow is balding.
Unless you are very careful, there are a 101 ways for you to accidentally catch some of your bow hairs. In between the strings near the qianjing area, the corners of the soundbox, the scales on the snakeskin, the strings wound around the pegs and the number 1 bow hair killing machine - fine adjustors. A bow with thinning hair gives a thinning tone and you should change it as soon as possible.
2) Compounds of rosin and sweat are accumulating on your Erhu bow hair.
Especially at the area near the ferrule where you place your right fingers. When the rosin reacts with the sweat on your fingers, it forms a sticky gluey substance on the hairs that is difficult to get rid of. You can try to wash it with non oily dish washing detergent though.
3) You get cackling harmonics when you play your open strings.
Once a customer came to my shop, complaining of a cackling noise when she bows that was not there before. I see nothing wrong with the Erhu bow, thus I changed the bridge and dampener and changed the strings but the noise still persists. Finally I changed the bow and the
noise is gone. Sometimes the rosin reacts with the moisture in the air and forms bigger particles among the bow hairs which you can't see. Again you can try washing your Erhu bow or live with this minor irritation. Somehow the cackling noise is more apparent when you play open strings.
4) Something ain't right but you can't put a finger to it
Sometimes you can't find anything wrong with the bow, but you just feel that its not the same as it was. Go with your instincts.
Don't wait until your bow becomes like this before you change:
Oh, and don't bother about rehairing your Erhu bow unless it has sentimental value. Getting a new Erhu bow is much cheaper than getting it rehaired.
And did I mention that we ship our Erhu bows in custom made postubes that absolutely prevents the bow from damage during shipping? Unless an elephant steps on it.
