Actually, if you can loop yourself more than six times at any small stretch of wall-time then you can get more than 30 subjective hours in one 24 wall-time day.

But it's implied you can't actually do that, which is why I think no more than 6 copies at any given time. Plus, if it were possible you could basically use any one day as a stopping point groundhog-day style in which you can (for example) brute-force read the entire Hogwarts library.

At any rate, the general limiting principle is that information cannot travel more than 6 hours backwards, Which I think means that when you draw a graph of a person using time-turners where you represent her using an arrow (going right for positive time, and left in 1h jumps for time-turner use), Then you can't have more than 6 hours of left-arrow in any given 24h wall-time section.