The act of sweeping produces friction and collects debris in the fabric. The result is the fabric on the brush head is subject to normal wear. With use, decline in a brush heads sweeping performance can be expected. Typically curlers outside of those competing in elite competitions have paid little attention to the condition of the brush head. The importance of doing so is now better understood.
The World Curling Federation (WCF) believes a good analogy is that curling brush heads should be treated more like golf balls, than like golf clubs. A golfer regardless of their ability wouldn't likely attempt to use the same ball for an entire season. Similarly, replacement of curling brush heads, pads or covers, like golf balls should occur on a regular basis.
Some manufacturers suggest curling brush heads should be replaced once a season if they're used once per week, twice per season if they're used twice per week, and so on. The WCF believes this to be a useful guideline for club curlers to consider.
Taken from the WCF web site.