What this video covers
The clip covers summer, when outdoor air is the moisture source rather than a leak.
Above roughly 60 percent indoor humidity, growth can establish within a couple of days. Air conditioning is doing dehumidification as well as cooling, which is why running it matters, and why opening windows on a humid day undoes the work.
The takeaway
In humid months the building should stay closed and the AC should stay on. Fresh air feels better and raises the humidity you are trying to control.
Read the full written guide: Summer Humidity Mold Prevention.