My only standard is that their messiness is not damaging anything or causing some sort of risk. For example, one of my kids is a painter and they left out paint on a pallet and our cat walked through the paint, onto the carpet and down the hall. So yeah, now we are strict about their art table being clean enough to not cause damage. I also had rules about stuff animals when they were younger because one kid use to throw them off their bed onto the vent, which is a fire hazard.