REAL plaster or joint compound??

real plaster

Depending on the ph-level/mix of that plaster, usually waiting a month or more is wise.
It takes a while b4 it's ready to prime.

Depending on final topcoat color...
Zinnser's Gardz primer would be good, along with their 123.

Faron

Not oil.
Veneer systems (Blueboard + 1 or 2 coats of veneer) claim to be paintable in less than a week.
Three-coat gypsum plaster over "lath" needs four weeks.
Real lime/horsehair plaster needs 90 days to cure enough before you should dream of painting it.
Beyond those calendars, feel it; if it is at all cool and clammy, it isn't ready for paint; that goes for drywall mud, too.
Casey

Hello, just closing the loop here for the sake of people searching for a clear answer in the future.

According to another paint forum, Sherwin Williams Loxon is the best primer to use for "new" real plaster.

better make sure to remove and wipe off any dust after sanding, or nothing will stick learned that one the hard way