I keep my toilet brush until it wears out which is usually around 6 to 8 years of use and sometimes longer.
There's really no need to replace your toilet brush until it wears out and is no longer useful for cleaning the toilet and you can pour some bleach in the bathtub or in a bucket and let the toilet brush sit in the bleached water for a few hours to keep it clean and disinfected.
Some people replace their toilet brush every 6 months to a year but I would say that's too often unless the toilet brush has become worn out and not effective at cleaning the toilet.
There's no need to replace the toilet brush every year or every 6 months unless it really does no longer work to clean the toilet.