The short answer
Allow four to six weeks for fresh plaster to dry before it sees any paint. In a cold house, or through a Dorset winter with the heating on low and the windows shut, it can take eight. A room that has been boarded and skimmed dries faster than one plastered onto old brickwork, because there is less water going into the background.
How to tell when it is ready
Fresh plaster is a deep, even pink-brown. As it dries it goes pale, and it does not go pale evenly — the edges and the middle of a wall dry at different rates. The wall is ready when the whole surface is a uniform pale colour with no darker patches left, particularly in corners, behind radiators and along the bottom of the wall where it meets the skirting.
If you can see a darker shadow anywhere, the wall is still giving up water. Paint it now and that water has nowhere to go.
What a mist coat is, and why it matters
New plaster is porous and thirsty. Ordinary emulsion painted straight onto it sits on the surface, dries too fast, and never keys in. It looks fine for a year, then peels off in sheets when you pull a picture hook out or catch it with a hoover.
A mist coat is emulsion thinned with water, usually around three parts paint to one part water, worked into the surface so it soaks in and gives the next coats something to grip. It must be a matt emulsion, not vinyl silk and not a one-coat product. Most of the "new plaster" primers sold as a shortcut are fine, but ordinary trade matt thinned down does the same job for less.
Speeding it up, carefully
Ventilation dries plaster; heat alone does not. Open windows, even a little, and keep air moving. A dehumidifier in the room helps. What does not help is blasting the heating or pointing a heater at the wall — that dries the surface while the depth of the plaster stays wet, and you get cracking and a skin that lifts later.
What happens if you rush it
Three things, in order of how often we are called out for them. Paint that flakes off in sheets. A milky, patchy finish where the paint has been drawn unevenly into wet areas. And staining that keeps coming back through the paint because salts in the wall are still migrating to the surface.
All three mean stripping back and starting again, which costs more than the four weeks you saved.
Questions we get asked about this
Can I put wallpaper on new plaster?
Not until it is fully dry, and the wall should be sized first. Paper hung on green plaster traps moisture behind it and the seams lift.
Is it ready if it looks dry after a week?
No. The surface dries first and the depth takes far longer. Judge it on colour across the whole wall, including corners and behind radiators, not on how it feels to touch.
Can I use a dehumidifier?
Yes, and it helps. Keep the room ventilated as well — moving air does more than heat.