This is the most consequential timing decision a homeowner makes on a plastering job, and it is almost always made too early.
The manufacturer's answer
Dulux state as a rule of thumb that plaster takes up to four weeks to dry completely, and warn that emulsion applied to wet plaster is likely to crack and peel off.
Four weeks feels excessive when the wall has looked and felt dry for a fortnight. It is a safe margin rather than a hard minimum, but it exists because paint manufacturers spend a great deal of time dealing with failures caused by early painting.
The test that actually works
Forget the calendar and look at the colour.
Wet plaster is dark brown. As it dries it lightens to a uniform pale pink. The wall is ready when it is that pink everywhere, with no darker areas at all.
Check the places that dry last: the corners, behind and above radiators, the bottom of external walls, and any area behind furniture. A wall that is pink across the middle and still brown at the edges is not dry. It is a wall with weeks of drying left in the parts that matter most.
Look at it in daylight from an angle. Raking light shows tonal differences that a ceiling bulb hides completely.
What happens if you paint too soon
The moisture in the plaster still has to leave, and you have just sealed its exit with an impermeable film.
It finds another way. Sometimes it lifts the paint into blisters. Sometimes it pushes the whole coat off in sheets. Often it carries dissolved salts from the plaster and brickwork to the surface, leaving brown or yellow staining that bleeds through repainting, or a white crystalline bloom of efflorescence.
None of these are fixable by painting over them. The remedy is stripping back to bare plaster, letting the wall dry properly, treating any staining with a stain-block, and starting again. The four weeks you saved cost you the whole decorating job.
Speeding it up safely
Ventilation, not heat. Open windows and move air across the wall with a fan. Moving air carries moisture out of the room; a hot sealed room just holds warm damp air against the surface.
A dehumidifier in a closed room works well because it physically removes the water.
Do not put heaters against the plaster or run underfloor heating hard. Drying the surface faster than the depth causes shrinkage cracking and a weak, chalky face that never takes paint properly.
South London's solid walls
Most of the housing here is Victorian and Edwardian solid brick. Plaster on a solid wall dries from one face only and the wall itself holds moisture, so the four-week figure is closer to a realistic expectation than a conservative one, particularly on chimney breasts and external walls in winter.
Modern plasterboard partitions dry much faster.
Then prime it properly
Dry plaster is not ready for full-strength emulsion. It is highly porous and will pull the water straight out of undiluted paint, leaving a patchy coat with no adhesion.
Apply a mist coat first, roughly ten parts paint to one part water, or use a proprietary plaster sealer. Then two full coats on top.