How long shd it take for walls in new extension to dry out?

We had a detatched games room built last winter. Unfortunately, there was a delay in the roof going on of about three weeks and the weather at the time was horrendous and very wet.

Initially the insulation in the roof was very wet but this has now dried out. However, whilst the walls have been painted and seem fine, when my husband screwed some pictures directly to the wall, they soon rippled and on removal had damp patches on the back.

We are guessing the damp is the water that would have got in the cavity during the wet weather when built which is still coming out and by putting up the picture he blocked its exit. However, we are only guessing and any advice from someone more knowledgeable would be welcome.

Just in case it helps, the roof does have breathable felt and we have since added soffit vents to get some airflow into the roof. We have also thought about adding some tile vents, although I don't think the damp in the walls is coming from the roof as this now appears dry.

Any advice would be very gratefully received - thankyou!!!

What are the walls made of, are they brick/block then plastered?
Timber frame and boarded?
Dot'n'dab?
or something else?

Thanks so much and apologies for the delay in coming back to you.

The walls are brick / block with plasterboard. I think the plasterboard was put on with dot and dab.

Hope this helps!