For nightly floor use on the Isle of Man, lift and inspect the underside each morning. The amount of airing needed depends on the bedding and base condition, not a fixed island-wide timetable. Arrange an indoor option for days when outdoor conditions are unsuitable.
What if the room is heated but the floor is cool?
Do not use room warmth as the only drying check. Look underneath and inspect the base. When either is damp, separate the two surfaces and allow them to dry rather than returning the futon to the same position immediately.
When should I turn it over?
Turn and change the head-to-foot direction regularly to vary the area receiving pressure. Pair that habit with actual airing. A quick flip is not evidence that moisture has escaped, and sunny conditions do not guarantee a particular drying time.