Use a clean container for each batch. Residue from a prior batch will accelerate the next one. Fold the mix, as in bread making, rather than stirring, which generates heat. Mix no further than necessary to remove lumps and spread the mixture thin in the pan to retard hardening. Adjust the size of the batch to however many molds you can fill before the mix 'kicks'. Adding vinegar will further retard setting. "Readiness" will depend on your particular application, but I would begin filling the molds ASAP. Much will depend on ambient temperatures, and cooler is better, so you need to experiment.

In the cement industry, add retarding admixtures or super plasticizers to acheive this effect.

Look at ASTM C494 types B & C to get some ideas. or ASTM C1017.

I know that Ammonia, hydroxylatedcarboxylic acids, lignosulfonic acids and various magnesium or borate salts are used in concrete.

Carbohydrates would also have a similar effect, High fructose corn syrup would be my first experiment, as long as I wasn't worried about feeding mold, fungi, or insects down the road on this product's end use.

milo

I have found that gelatin works very well. I have also been told that sugar can do a good job.

The other thing I have used is lime. This is in line with the impasto used by Leonardo and his mates. Lime converts more slowly as it has to react with the CO2 in the air to harden. The Gypsum hardens by direct crystallisation with the water and is much faster. I used a mix to give me a pattern on a wall. It is a case of suckitandsee - that was how I did it!.

To give a nice finish try adding white wood glue - this might also work.