When
skimming on top of old plaster or plasterboard, you should wait
3-4 days before applying a sealer and then normal 2 coats of paint.
If your room has been plastered from the brick, the normal drying
time is 6-7 days.
In both cases, it will help if you leave windows open ... new
plaster will take on a light pink colour which tells you it's
ready to start paint.