AHU shutdown for No's of Hrs. time validation

Dear All,
If AHUs are shutdown due to electricity or any other reason for xyz hrs…then how long area can be consider for further use…
Is there any time validation for AHU shutdown ?
In our SOP fumigation is on weekly basis and as per requirement of area results of environmental monitoring.
In this case after how many hrs AHU shutdown fumigation is required?

Please provide the reference guidelines​ or process for this