New plaster goes through two different processes and people conflate them. It sets within a couple of hours through a chemical reaction, and it dries over days or weeks as water leaves the wall. A wall that is hard to the touch can still be soaking wet behind the surface.
The honest timescale
A 3mm skim coat over sound plasterboard is normally touch-dry within 24 hours and properly dry in three to seven days in a warm, ventilated room. Backing plaster applied at 8-11mm on brick takes considerably longer, often two to three weeks. Dulux, whose commercial interest is in you not painting too early and blaming the paint, states plainly that as a rule of thumb plaster takes up to four weeks to dry completely.
Those two figures are not in conflict. Three to seven days is when a thin skim stops being wet. Four weeks is the safe margin before you seal the surface with paint, which is a different and stricter test.
Judge by colour, not by date
This is the part worth internalising, because it is the only reliable method available to a homeowner without a moisture meter.
Wet plaster is dark brown. As it dries it lightens to a uniform pale pink. Patchy plaster, dark in the corners and light in the middle, is not dry, no matter how many days have passed. The corners, the area behind a radiator, and the bottom of an external wall dry last, because that is where air moves least.
Look at the wall in daylight from an angle rather than head-on. Darker patches show up far more clearly in raking light than under a ceiling bulb.
What South London housing stock does to the timescale
Most of the area is Victorian and Edwardian solid-wall terraces. Solid brick holds moisture in a way a modern cavity wall does not, and plaster applied directly to it dries from one face only. On a chimney breast or an external wall in a Camberwell or Peckham terrace, expect the longer end of every range quoted here.
Flats and newer builds with plasterboard partitions dry much faster because the board itself absorbs some of the water and both faces are exposed to air.
Speeding it up without ruining it
Ventilation beats heat. Open the windows, even in winter, and use a fan to move air across the wall rather than at it. Moving air carries moisture away; a hot, sealed room simply holds warm damp air against the plaster.
Do not put a heater directly against fresh plaster or run underfloor heating at full temperature. Forcing the surface to dry faster than the depth causes shrinkage cracks and, in bad cases, a chalky surface that never takes paint properly.
A dehumidifier in a closed room is the exception and works well, because it removes the water rather than just warming it.
Why this matters more than it sounds
Almost every dispute we hear about plastering is really a drying dispute. The plasterer finished, left, and the homeowner painted eight days later because the wall looked fine. The emulsion sealed the remaining moisture in, and within a month it was peeling, blistering or showing brown salt stains.
That is not a defect in the plastering. It is a defect in the timeline, and it is nearly always the homeowner who created it. If you take one thing from this page: the wall is ready when it is one even colour, and not before.