Ideally one would use the joint compound with the desired setting time, together with the recommended amount of water for mixing. However, a few times I added more water when mixing products like SheetRock Easy Sand 5 Lightweight Setting-Type Joint Compound to gain longer working time, the result turned out fine. I added enough water until it is almost runny. The mix probably remained usable for 20-30 mins. I'm curious what may be the downsides to this practice.
Adding water to prolong working time is known as "retempering". In general it's not a good idea as it dilutes the concentration of cement components in the mix. This means that the resulting material is more porous. It's often specifically prohibited with things like tile mortar, where strength is a critical concern.
In the case of joint compound it's not so serious a problem. However, unless you mix vigorously you'll start to see lumps in the mix as it sets up. Your best bet is to mix small batches, use it within the stated (or actual) working time, and toss what's left. Use cool water to slow the curing reaction. Warm water dramatically shortens working time.
Most of the compounds I've used usually say you can a little more or less water depending on what the application is for. Like Stove Top Stuffing: add a little more water for more moist stuffing or less for dryer stuffing. If you add too much water to your compounds, then you'll end up with compound soup and it just won't stick to the wall.
Cement is kind-of "the first epoxy". It's 2 separate ingredients coming together to create a third: namely portland cement and water.
Epoxies have 2 things going on: Drying, and curing. These are separate processes. They are not related, and each one runs on its own timetable.
So if someone is "a little bit clever", they might find a way to keep it workable (un-dry) despite the fact that the curing process is running its pace. The problem is that the point of curing is to create chemical bonds, and you are breaking those bonds when you continue to work it. Thus, you will have fewer bonds holding it together, and the material will come out weaker.
Such materials cure on sort of a "half-life": Half the cures happen in X time, and half the remaining cures happen in the next X time, etc. etc. So the curing is "front-end loaded", and disrupting the cure early takes out most of the strength.