Product review (4 stars): Change it to keep healthy
Paranoid about my humidifier growing bacteria and making my family sick.  I make sure to keep the filter dry when it's not running enough like when it gets too humid.  I also use a anti-bacteria liquid with every other filling to kill off anything that could have grown.

I figure the first filter lasted about a quarter or 3-4 months.  It gets so crusty after a while it's clear you need to change it so I go by looks to figure out when to change.