I'm not comvinced at all by this thread regarding the frequency of brush replacement.
The brush doesn't really do the cleaning, the water does that and the rinsing afterwards.
All the brush really does is agitate the surface of the glass to get the dirt moving.
Provided the bristles aren't falling out or worn where is the problem?
I'm sure the people who want to sell you a new brush every six months or so will tell you a different story.
But my three year old brush works just as well as it ever did and I have absolutely no plans to change it.