It should be as dry as possible, you can feel it in the air if there's a lot of moisture in the room, maybe a week and then it's dry, then start by lightly running a wide putty knife over the wall (there will be a lot of small gravel that will come loose) then apply one layer of putty, once more with the knife over it to remove anything loose and bumpy, a second layer of putty and then sand it lightly. You will need to apply putty at least twice.
