Grab the low hanging fruit first. Grab all appliances first. Get them out of your way. Next do the plumbing and plumbing fixtures. Next would be electrical. Start at the outlets. Smash a hole in the walls above the outlets and see which way the wire runs. Cut wire at the outlet and pull so it pulls through the plaster or drywall. It will save you time instead of pulling down all the walls. You may want to bring a small generator to hook up some drop lights to if you have to cut power before starting the job. If you only have 20 hrs that is what I would go after. Get as much of the non ferrous first and grab the steel last. Good luck.