Every time you play you are stretching the hair. The longer you practice the more stretched out the hair becomes. Factors such as high humidity combined with the stretching will make the hair expire rapidly. There is also the occassional, "oops, I left the bow tightened overnight" mistake. All that will result in needing fresh hair more often.

Under everyday use, hair stretches and loses its rosin holding qualities in about 6 months. After that, you are on borrowed time. If you play and practice about 2 hours a day or less have your bow rehaired once a year. If you play and practice 2 to 4 hours per day or more, bring your bow for rehair every 6 months.

If the Hair is as dirty as this bow is, you also need to renew the hair.

Oils from our hand combined with rosin will coat the hair with greasy looking residue. Washing your hands before playing is always a good idea.

If the Hair is not sagging below the stick, but your hair is dirty and your last rehair was only a few weeks ago, the hair may only need professional cleaning.

Notice how much hair is missing on the edge of this bow.

That is a very good sign the hair needs replacement.

Using the bow with hair like this causes uneven tension on the bow and may begin to warp the stick.