From what I've read and experienced, I think it's the amount of time between the 'resets' that is crucial, not the total time of having them on. 'Resetting' as in, taking them off for a minute or so then replacing them. That would logically solve the circulation issue.

I've had them on for a total of 2.5 hours recently with resets every 20-30 min. It hurt like the dickens, but they were perfectly fine after. If you try them for 6 hours or something, though, I figure you might have some abrasions on the nipples or the like?

This only applies to normal plastic clothespins, not the severe kind of clamps.