Ideally, the plaster should be completely dry before painting. Fresh plaster will take 4 to 6 weeks to completely dry. Fresh plaster is porous and will therefore absorb moisture. If you try to paint on plaster that is still damp, the paint is likely to peel off.
If you try to apply paint to wet plaster, this can cause adhesion problems. Applying emulsion to wet plaster also means that it may not adhere properly and may come off the wall. Plaster damage is not as easy to repair as damage to drywall or drywall, which comes in solid sheets that can simply be cut and fixed to the wall or ceiling. For example, if the cause of the damage is a slow leak or a broken pipe behind the drywall, you are likely to suffer continuous damage until you repair the cause of the leak.
Another reason why you should repair damaged plaster before painting on that damaged stain has to do with the paint itself. If the damaged part of the plaster is just a water stain and there is no underlying leakage that recurs, use a primer that blocks stains, such as a Kilz sealant.