Having listened to the various tales of woe regarding using old plaster, I wondered how old is "too old"?
Having just purchased seven bags of multi-finish and humped them up two flights of stairs, I popped out to the workshop to collect the plastering tools. Happened to notice that there was still a 1/3rd of a bag remaining that I had opened a few weeks prior. Thought, its not been there that long, should still be OK, might as well use that first.
Mixed to a nice creamy texture, I deposited it on a spot board and went to wash out the bucket... came back five minutes later to find a solid mound of plaster glued to the spot board! Guess that answers that question!