Fresh plaster looks ready long before it actually is, and painting too soon is the most common reason new walls peel, flake or stain within months. The honest answer on drying time is a range, not a single number, and on the Wirral peninsula the weather and the age of your house both push that range around. Here is how to judge it properly.
How long you wait depends mostly on how thick the new plaster is and what it has been applied to. A thin skim over plasterboard has very little water to lose. A full replaster onto old brickwork holds far more, and the wall behind it soaks up moisture and releases it slowly.
A widely used rule of thumb for thicker work is roughly one day per millimetre of thickness in good drying conditions, and longer when it is cold or damp.
Wirral sits between the Dee, the Mersey and the Irish Sea, so the air carries more moisture than it does further inland. In places like West Kirby, Hoylake, Meols and New Brighton, a damp, still week in autumn or winter can add a week or more to drying time compared with a breezy spell in May.
House type matters too. The Victorian and Edwardian terraces of Birkenhead, Wallasey and Oxton mostly have solid brick walls with no cavity, which hold moisture and dry slowly, especially on cold north facing elevations. A 1930s semi in Bebington or Bromborough with cavity walls will usually dry faster, and a modern plasterboard extension in Heswall faster still.
Rooms at the back of the house, chimney breasts, bay windows and anything below ground level are the usual slow spots.
Go by the colour, not the calendar. Wet plaster is a dark brownish pink. As it dries it turns a pale, even, chalky pink. The wall is ready when every part of it is that same light shade, with no darker patches left.
Check the places that dry last: internal corners, the bottom of the wall near the skirting line, around sockets, and anywhere the plaster was laid on thicker to level an uneven wall. One stubborn dark patch means the whole wall waits.
To help it along, keep windows on the latch for steady ventilation and run the heating at a low, normal background level. A dehumidifier is fine if used gently after the first few days. Avoid blasting the room with fan heaters or turning the heating right up, because plaster that dries too fast can crack or lose its bond with the wall.
Bare plaster is extremely absorbent. Put standard emulsion straight onto it and the wall sucks the water out before the paint can grip, leaving a skin that peels off in sheets later. The fix is a mist coat, which is simply watered down emulsion that soaks in and seals the surface.
Paint tins vary, so read the label, but these are sensible working figures:
Paint over damp plaster traps the remaining moisture behind a film. The usual results are blistering, flaking, patchy colour that never evens out, white salty deposits pushing through, and sometimes mould. Putting it right means scraping back and starting again, which costs far more time than the extra week of waiting would have.
Avoid PVA as a sealer under paint as well. It sits on the surface rather than soaking in, and emulsion often fails to bond to it. Wallpaper and tiles need the plaster fully dry too, and tiling may call for a specific primer, so check with your tiler.