The plasterer will likely use a calcined-gypsum product....A 'setting-type' gypsum 'joint compound mix...with plaster added to enhance fast setting.

This product usually comes in 15, 30, 60 and 90 minute "pre-mixed" setting times if I remember correctly.

The ratio of plaster (a lime based compound that hardens by chemical reaction) to gypsum (a 'mud' made of gypsum and water which dries by evaporation) determines the time one can apply and finish the mix before it chemically hardens.

The cost of materials in such a case is minimal.

Finding a competant contractor who can do a superior job within the setting limits of the product is the challenge.

"The ratio of plaster (a lime based compound that hardens by chemical reaction) to gypsum (a 'mud' made of gypsum and water which dries by evaporation) determines the time one can apply and finish the mix before it chemically hardens."

Setting type joint compound hardens by chemical reaction (the same as plaster) and not by drying.

There would be no reason to add plaster to a setting type compound.

Gauging plaster is mixed with retarders or lime putty to slow the setting time, snce it is very short (less than 10 minutes) and not workable for finishing without being slowed down.

The combination of lime putty (weeks to months to fully sure) with plaster produces a reasonable setting time.

If you have a bag of Durabond 90, and a bag of gaging plaster, you have the ingredients to make a setting-type joint compound with a setting time from 90 minutes to 1 minute (goes off in the mud pan!) DAMHIKT!
If you're doing one small repair or area, you can "enliven" the durabond by adding in a little or a lot of gaging. It will set as fast as you can handle it.
Anyways, just because the durabond is "set" doesn't make it dry enough to finish! The water still is there, and has to process itself out before it can be painted. Rule of thumb: if it feels cold and clammy, it's not ready for paint even if a "cat couldn't scratch it".
Casey