The working time is a bit longer than that
it's a race against time to decorate the interior of the box before the plaster stone begins to set. (5 minutes per box)
Oh I didn't know that it was that fast to harden, that's really incredible that you are able to make something that pretty in such a short period of time. I'm impressed.
The working time is a bit longer than that, but the surface starts to dry out sooner, so it has to be done very quickly. The largest items go in first, and the sands and particles goes on last.
What's probably more interesting, is that if you look at the order of steps, the whole thing is done blind—the paper masking isn't taken off until much later, so I don't even know what it looks like from the front or side views until the very end.