The thickness of the duct  is determined by the length of the long side of the duct (the diameter  of the duct is the diameter) and the working pressure of the duct:
Low pressure system: P ≤ 500Pa
Medium pressure system: 500Pa <P≤1500Pa
High pressure system: P> 1500Pa
To low pressure, galvanized iron duct thickness as an example:
Main length of air duct b ≤ 320mm: 0.5mm
320 <long side of the duct size b ≤ 630mm: 0.6mm
630 <long side of the duct size b ≤ 1000mm: 0.75mm
1000 <long side of the duct size b ≤ 2000mm: 1.0mm
2000 <long side of the duct size b ≤ 4000mm: 1.2mm
These are from the "ventilation and air conditioning construction quality acceptance GB50243-2002"