Depends on the room temperature, and if the walls were bonded first or just skimmed.
As new plaster is very dark when wet, you should see it getting lighter as it dries. Wait till all the plaster is the same light colour before decorating.
If the walls were skimmed it will probably take a just few days, but a bonded wall will take longer as there is a greater thickness of plaster and alot more water!!
For painting, use a dilute coat of emulsion first to seal the plaster - 1part water and 3parts paint is fine.
For papering, 'size' the walls first. It can be done with a watery wallpaper paste or size, they both so the same job.
Once sealed, decorate according to taste!!
If the new plaster is not sealed first it absorbs the emulsion too fast and you end up using twice as much just for one coat, and pasted paper may not go up properly!!
A word of caution on room temperature- if the room is too hot the plaster will dry too fast and you could end up with loads of cracks!!
I've had to have every room in my house done and have learnt from bitter experience, it's better to wait than rush it!!!
Good luck, hope it goes ok.