How to calculate the time needed for the UVC to be turned on and disinfect a specific space/location?
Dose Requirements are usually given in uJ/cm2, mJ/cm2, uW/cm2 or mW/cm2 and are not in power per covering radius and exposed time. So, it is the flow that must be taken into consideration for air treatment and you must consider recycled airflow treated by UV or treating air flowing inside air ducts like HVAC elements.
As vague measure it is noted in this reference that the ideal UV-C intensity for effective disinfection must be greater than 10uW/cm2. But it depends also on the type of virus or bacteria you are fighting.
In all cases you must also consider the efficiency of the UV tubes.