I've been cleaning house today and find cleaning an interesting idea because it's something that needs to be done periodically, like eating, but not all the time, like breathing. In other words, the two extremes of cleaning are both useless-- cleaning never means that cleanliness does not exist and cleaning always means that not much else exists.
The real question, then, is how often does cleaning need to be done? A sensible answer would be, "when something's dirty," but that's subjective since dirtiness is a gradual scale, not an on-off switch. As far as I can tell, the college student answer is that cleaning should be done when the discomfort of things not being clean overcomes the discomfort of cleaning. This is far from a solved question, but at least it gives me something to think about while wiping down windowsills.