The reason for not painting before the wall is completely dry is that most paints will simply form an air tight skin over the wall. The moisture from the new plaster is then trapped behind this skin and cannot evaporate off. A fully plastered wall, i.e. two or even three coats of plaster, can take months to dry. There are paints on the market called Microporous Paints. These paints allow the surface of the plaster or artex to carry on breathing and evaporating while they are drying out. If you intend to put an oil based paint on the surface, such as eggshell or gloss, then the surface can be sealed using a solution of PVA adhesive. This is mixed at the same ratio. Some companies actually produce a plaster sealer.