A used filter is anything that has come into contact with water. So, whether you use your filter every day, or you only use it at work or in a vacation home, the moment the filter touches water, it becomes used.
You should replace your filter as recommended by the manufacturer regardless of how often you use it. This is because, once the filter becomes wet, it becomes vulnerable to bacteria growth. Drinking bacteria can make you sick, and getting sick from your water would go against the purpose of using a filtered water solution.