What is the best time for duct cleaning?

There is no best time for duct cleaning but EPA has certain guidelines to explain when your ducts need cleaning. So you need duct cleaning when:
You haven’t got it cleaned for a while.
Any animal has intruded your ducts.
There is mould growth inside the duct.
There is dust coming in the air.
The duct does not work efficiently.
You feel any change in duct airflow.
Your power bills increases for no reasons.