Good question, but to answer that question we must first ask an even more frequently asked question.
How often should I change my filter?
The answer is.....it depends. What a let down.
General rules of thumb:
If you have standard 1” pleated filters you should be changing them once every 30 Days at least!
4-5” Pleated filters should be changed every 3-6 months.
Electrostatic filtration systems (less common) should be changed and cleaned according to manufacturers recommendations. Usually pre filters should be cleaned or changed every 30 days and internal components should be cleaned every 3-6 months.
Electrostatic filtration systems usually have a contamination meter to indicate build up.
Here is the catch...(there had to be one right)
If you find that your filters are clean at normal change intervals with normal use that may indicate an air flow related issue within your system usually caused by undersized intake grills.
The general rule of thumb for grill sizing is 200 sq inches per ton of cooling capacity. For example if you have a 4 ton system you should have 800 sq inches of return grill space equal to two 20x20 intake grills.
A sizing tool to use to find out if you have enough return air grill space is measure the height and width of the louver space (the open cut space in the grill) and multiply them together to get the total square inches of the grill. For example a 12x12 return air grill will equal 144 sq inches of return space.
In conclusion, having the proper amount of return air grill space is essential to getting proper air filtration. Undersized grills will most certainly lead to dirty coils and redistribute dirt and dust through your supply air system!