Most people recommend changing your toilet brush to a new one as often as 6 months to a year.
But to me that seems to often because toilet brushes often last me longer than that and sometimes as long as 2 to 3 years of use.
I could see myself replacing a toilet brush or toilet brushes more often if they were used by employees in a commercial setting cleaning several toilets daily with that same toilet brush.
But for home use you should not need to replace your toilet brush all that often as long as the toilet brush still works and has all the bristles on the toilet brush it's perfectly okay and safe to use it until it wears out.
I usually get at least 3 years out of a cheap toilet brush before replacing it.
So you can replace your toilet brush as often as you like but there's not need to waste the money on a new toilet brush if you don't really need a new one.