1" Wide throw-away filters tend to last anywhere from 1 - 3 months. 3-6" wide filters will generally last 6 - 9 months.
Many service calls could be avoided if the filter was changed earlier. Often, a filter is so dirty that it impedes airflow enough to shut the unit down. Sometimes, it will be enough to ruin a blower motor.
We generally ask that you perform the filter change yourself due to the required frequency. However, don't hesitate to call us with any filter related questions. If needed, we can certainly build semi-annual or annual filter changes into a contract.