New plaster can generally be painted anywhere from 4 to 6 weeks after the wall or ceiling is first completed. If the plaster work is done in a building with an additional heat source other than standard central heating, the plaster can set in as few as 3 weeks.
However, if there are multiple layers of plaster, it potentially takes a couple of months to dry completely. It is imperative to let the plaster fully dry before painting. Applying a coat of paint to still-damp plaster can cause mold to form. Even worse, it creates the potential for efflorescence, which is a salt deposit that forms on the outer wall.