Can you say what kind of plaster? Also, are we talking plaster or are we talking drywall and drywall mud?

I am not sure of the kind of plaster. It is full wall and ceiling, all over plaster. Plaster covered the drywall and mudding.

Usually the plaster is primed, otherwise it'll soak up an huge amount of paint (paint is more expensive than primer). I've seen both acrylic and oil primers used (not at the same time, of course). I've also noted that some painters will do an initial coat with a thinned primer and then a second coat of the undiluted primer. It's also important to ensure the plaster is completely dry before painting (I think this can take up to a couple of months, depending on thickness).

What are your opinions on paint mixed with primer?

How do builders work around the drying phase? It seems unrealistic that they would even consider waiting that long for the plaster to completely dry. That is where we are getting mixed opinions on what do to. We just want to make sure it is done correctly.

I would contact the company who did the plaster and ask them what they recommend. If anyone would know, it would be them. And since they are the ones who would be most interested in whether or not their work is compromised by another trade, I would follow their recommendations. Painting too soon or with the incorrect material could damage work already done and set you back in time and cost.

Thanks for your help!