Plaster is generally not used for its compressive strength qualities, so
there is not likely to be much reference data available.
There are numerous formulations of plaster: gypsum, lime-gypsum, portland
cement-lime. Your specific formulation will probably need to be tested to
establish a strength.
If you are working with an existing plaster, you may not be able to cut a
testable specimen out of it. However, it may be worth a try and you may get
some useful information.
U.S Gypsum plasters have various constituents; some, like their structo-base
are very hard -- equivalent to concrete-- but U.S Gypsum will not provide
any strength data on it.