Dan_Musicant writes:

wonder if it will last longer if I
store it in air-tight containers and store in a cool location.

Plaster will strike in the bag due to infiltration of humidity. It will
still be a powder that mixes up OK, but given enough storage time and
humidity before use it will have a final strength of a Ritz cracker.

I would think the cost of proper containers would approach the cost of the
plaster.

Most cost effective would be 5-gallon paint buckets with the O-ring lids,
with a sock full of calcium chloride as a dessicant.

Easy enough to do your own "lab" testing later to verify setting quality.
Make some cylindrical samples (cast in a pill bottle) now and when you plan
to use it. See if you find any difference in strength or setting time.
Better than wasting a lot of labor on material that turns out to be flawed.

Steady non-condensing relative humidity is more important than cool.
Warmer may make your RH lower.