I think this is something that is impossible to lay down strict rules for. "How To" manuals usually say that if a limb becomes blue or purple or cold to the touch, that's too long. Others say no more than half an hour, but that seems a little extreme to me (probably written by a vanilla sex adviser, like Alex Comfort).

Really, you have to gauge by the effect of the bondage on the subject. Some people are happy in bondage, while others (raises hand) get claustrophobic.

Of course, some positions are five minute killers while others are comfortable enough to sleep in.

In other words, there are too many variables for a meaningful answer -- tightness of bondage, material used, position, location, psychology and physiology of subject, and probably some others as well.