After a night on the floor in Guernsey, lift the shikibuton and check the underside and the surface below. Let both dry before laying it down again or storing it. An outdoor airing opportunity is useful, but the morning check should not depend on having one.
What if there is no convenient dry outdoor window?
Raise the bedding so that air can reach the part that was against the floor. Check the floor too, rather than only turning the top face towards the room. A warm bedroom, or a dry-looking upper surface, does not establish that the contact side is dry.
How do weather records help?
The Met Office publishes location-specific climate records for Guernsey Airport. They describe outdoor conditions, not the moisture in your bedroom or futon. Use the room's actual ventilation and underside condition to adjust care; there is no island-wide drying time that we can promise.
Why can dampness remain underneath?
Perspiration and water vapour can enter the bedding during sleep. Contact with a solid base limits the airflow underneath; condensation may also occur when the surface is cool enough for the conditions. Turn and rotate the bedding regularly, and store it only when dry. Airing cannot permanently restore filling that has compacted over years.
Climate reference: Met Office: Guernsey Airport climate records.