Between 0 - 3 times a day but never usually more than a couple days without refreshing the air, after 4-7 days I leave an air gap.
If I over-water or it gets too humid I will leave an air gap or refresh the air many times a day. If I have a fine mist of condensation over about half the dome it's in the happy place, if the whole thing is covered and dripping from the top of the dome it needs air.