until it's dry.
modern gyspum plaster is dark coloured, sometimes shiny brown, when it goes on, and dries to a matt pink. Sometimes, e.g. if there is a thick bit over a hole, or a wet brick, or damp due to a leak, the brown patches linger on for a week or more. You can't paint them as the paint will not dry.
you will easily see the difference.
Ceilings dry in next to no time.
Houses dry by fresh air, not heat, so open the windows and the loft hatch.
If the walls themselves are damp, which is common on a newly built house where the walls were exposed to rain before the roof went on, use a porous, breathing, non-vinyl matt paint. Dulux Trade Supermatt is best-known. Other paint makers may offer one, but examine the label or contact the manufacturer to confirm. You can order colours to be mixed, but white and magnolia are off-the-shelf and cheapest.
It is not very durable and will not withstand scrubbing, but makes the house look decent and you can paint over it once the wall is dry. Takes six months to a year. All the time the walls are drying, you will get condensation mist on your windows in the morning.