How to slow setting of plaster of Paris?

We are rookies learning to plaster some painted cinder block walls. A friend (who has done this many times) showed us some basics and we are managing along. Our problem is that plaster of Paris sets (not sure if that is the correct term) too quick. Meaning the plastic-like usable stage where we can apply it to the walls is about 2 minutes before it gets too hard to use any more. After about 3 or 3 1/2 minutes, the plaster is the consistency of cold clay (as used for pottery). A lot of time is wasted in mixing up small batches and if we make too much, some of is wasted because it hardened. We watched a youtube video of a British guy who mixed up 1/2 a 25lbs bag in a bucket. It takes us about 5 hours to get through one 25lbs bag. He had bigger tools, but even at the rate he was applying it, I don't see how he could use all that plaster up in 3 minutes or less. Unless he had other guys helping him that weren't visible on the video or was using a different kind of plaster. Google searching found some 20-minute setting plaster, which I'm uming would give longer workability time, but this stuff is not at our local hardware stores. Are we using the wrong kind of plaster for this sort of job? The walls are not that big, the largest is 14 feet by 8 feet. Any suggestions?