Here you can find out when you should have your bow restrung

On average, professional musicians have their bows restrung every 3 – 6 months.
Amateur musicians, who practise for about an hour a day and play in an orchestra or quartet once a week, every 1.5 – 2 years.
You realize that it is time for a new cover when

-Your hair no longer grips properly, even though you colophonize normally.
-You tend to use more rosin so that the bow grips.
-The hairs become longer over time and, in extreme cases, you can no longer draw the bow properly.
-The bow has already lost too much hair.
-The cover looks very dirty and greasy.