When to paint new plaster is never a simple question but in reality you should leave it for as long as possible!
If a wall or a ceiling has merely been skimmed with a single coat of finishing plaster then it may only take a couple of weeks to dry completely but if the wall has been plastered with backing plaster such as bonding plaster or browning plaster then it could be six weeks or longer before the plaster can be painted over!
Do not be tempted to seal the surface with diluted PVA as this can seal in moisture and lead to problems in the future.
There have been occasions where the galvanised metal beading used on all external corners has rusted and bled through the paint so for this reason I always treat the corners with a coat of stain blocker before applying a couple of coats of emulsion on top.
If you really must paint the wall before you are sure it is dry enough to paint then I would recommend that you use Dulux supermatt as this paint is breathable
Dulux fast matt can also be used as this can be applied just 24 hours after the plaster has been applied