New plaster needs to be left for at least a month to dry out. If salt crystals (efflorescence) appear in this time then brush them off.
The plaster needs to fully dry out before any wallpaper is applied, this can be up to six months. New plaster is also porous and will absorb moisture from the wallpaper paste, making hanging wallpaper difficult.
New plaster needs sealing (sizing) using a PVA adhesive mixed one part PVA to five parts water. An alternative method is to paint the walls with emulsion, mixed four parts paint to one part water.
Emulsion allows the walls to breathe and continue drying out. This also makes the room more liveable until it is ready to paper.
If you are going to cover in a vinyl wallpaper use a fungicidal paste or size to prevent mould growth.