You will immediately know when you need cleaning. For example, there will be a smell of dust or mold, you will notice a decrease in the power of the air flow, and there will be a noticeable accumulation of dust on the grilles. I once suspected that the system in my condo needed cleaning, and turned to the specialists of condo air duct cleaning . They came, conducted an inspection and showed me before and after photos. This is a great way to understand that it is time to act, even if you are not an expert.