PE mentioned this in the emulsion forum here - "once you add the hardener, the clock is ticking". I consider glyoxal in gelatin to be one-shot, but I measure glyoxal in drops and just use an eye dropper. In my experience, it doesn't need to be insanely accurate measuring.
I've been spraying Foma liquid emulsion onto canvas, and I use the included hardener (I assume it's chrome alum?). It takes three or four coats to properly coat, and the emulsion has to dry pretty well between coats, so it's maybe 30-40 minutes overall; I find in that time period, the emulsion still sprays well if kept warm, the hardener isn't setting up yet. I do clean the gun between coats though. But I dispose of leftover emulsion when I'm done, hate to toss it but my assumption is it won't be workable days later.