Don’t trust us for this question. As per EPA guidelines your duct needs cleaning when:
It has not been cleaned in a while.
An animal has entered the duct.
There has been a change in the airflow.
You notice some kind of noise or dust coming out of the duct.
Your power bills rise for no reason at all.
You see mould growth inside the duct.
The duct does not work efficiently.
It is best to turn on your duct before the start of the season to see if it needs professional cleaning.