If the A/C duct are not cleaned, they is a chance that they contaminated with debris,dirt, molds and mildews and even If there is any moisture, the chances of mold and mildew growth increase. Since air ducts are bring the air into your house, contaminated air ducts will release the spores in the living space.
I recommend to clean you air duct at least once every three years.
if you have any question I will be glad to answer them here but call me if you need help on cleaning your air duct.
Pete
Phone :309-530-9897