The effects of the shower filter have reduced
The shower filter is designed to last 10,000 - 12,000 gallons, or approximately 6-8 months of normal usage.
If you have a large amount of people using the shower or take long showers, then this time may be reduced.
You can calculate the lifespan of the shower filter as follows:
10,000 / (Number of showers per day x Duration of shower x GPM of shower head) = Total number of days
On average, Americans take one 8 minute shower per day, using a 2.4 GPM shower head.
Additionally, the chlorine level of your water plays a part in the lifespan of the shower filter. If you have high levels of chlorine, then the lifespan of the shower filter may be reduced.