Unless you have a very well sealed room it will always pull an amount of water out of the air as the air is being replaced by the damp(er) air from outside.
As long as you don't have 75% + humidity in the room it's 'dry'.
That's the hard bit though isn't it? Even when there was water trickling down the wall and i could squidge the plaster with my little finger, it was 70% humidity...
Unless you have a very well sealed room it will always pull an amount of water out of the air as the air is being replaced by the damp(er) air from outside.
As long as you don't have 75% + humidity in the room it's 'dry'.