That's an interesting question!
They are organic beings, but they are really not used to anything outside of their connection with Prime's hive mind and the nutrient plug-ins.
Wrong Hordak figured out how to eat actual food, so it appears that they are adaptable.
My guess is that until they actually "learned" how to sleep, they would just keep going until their strength diminishes to the point where they just pass out where they stand.
But eventually, I am sure they would get the hang of it, begin to see the signs of a need to sleep, and then figure out how to do it more gracefully.