I would be uncomfortable with set 'rules' on this subject, as opposed to guidelines. An 'only' cat will get bored and destructive if left alone all day every day, but the point during the day when that behaviour kicks in will vary on the nature of the cat. Some will just go 'humph' and curl up for a sleep.
If a cat has companion animals with whom it has a good relationship, then they can be alone for extended periods, even overnight, with no great harm done. The risk arises if there is an injury which is less likely if they are confined indoors, but still possible.
My lot have always had indoor/outdoor access when I am away from home (for work or social reasons) and all three (dog and two cats) are boon companions, so while the dog will become anxious if there are strange noises outside (e.g. the neighbour's dog has had puppies that yelp and whine all day) and become mildly destructive, the cats simply curl up and have a good snooze, or keep themselves happy hunting mice.
So, guidelines rather than rules, based on your knowledge of the temperament of the cat, and the lifestyle of the intended adopter would be my approach. The Brisbane RSPCA has a questionnaire that you are required to complete before adoption of any animal which goes into your lifestyle and habits fairly closely (e.g. amount of exercise, time away from home, family members etc). I have always passed this sort of test, so am unsure how strictly they enforce things if in doubt though ...