Depends on the brush.
I have been useing a gaza brush for 15 months and it still looks fantastic, their as been no wear and it as maintained its shape.
I have replaced my minipole brush after 13 months, it was an oval vikan, the bristles on the both ends or corners and become really short which made it more difficult to get into the corners of the windows.
I may replace a cleaningspot brush which after 6 months of use looks like its mind melded with Uri Geller, its totally lost its shape at the ends.
Nel.