Start with run time
A unit used most nights moves more room air than one switched on occasionally. Check the removable filter more often when use increases or when open windows, pets or household dust change the room conditions.
Use the morning check
After the system has run overnight, note unusual odours, weak airflow, water where it should not be or visible buildup around the louvres. These observations help determine whether the unit needs attention.
Keep the response proportionate
Routine filter care may be enough when the system is otherwise clean and operating normally. Persistent signs are a reason to arrange a closer inspection of accessible internal components.
The practical takeaway
For a high-use bedroom unit, regular observation is more useful than waiting for a major change in performance.