This should always be allowed to dry thoroughly before it is decorated, unless you are going to use emulsion paint designed especially for new plaster. Unlike vinyl paints, this will allow the moisture to continue to evaporate.
The time needed for drying depends upon its thickness as well as the warmth and humidity of the room.
As plaster dries it changes colour from dark to pale pink. When it is dry, any solids which appear on the surface (efflourescence) should be removed. A good way of doing this is to rub the surface with an old piece of hessian.
New plaster will need to be primed. The type of primer used will depend upon the final decoration required. If you plan to use oil-based paint, an alkaline-resisting primer should be used. For water-based paints, a diluted first coat is used to prime.
For wallpapering, the new surface will need to be sized. This is done either with size, or by using a diluted version of the wallpaper adhesive. It is usually best to use an adhesive containing a fungicide. This is particularly important if you plan to hang vinyl paper since it effectively seals the surface preventing it from breathing.
The preparation required will depend upon the condition of the surface. If it is clean, dry, and sound it will require no special preparation.
However, if the surface is powdery or there is other evidence of old distemper residue, it should be dusted off and sealed with stabilising solution.