We've had these conversations in the past. I think it's better suited for a number of our mega threads which can be found in my signature.
General consensus has been any longer then 2-3 seconds is simply depleting your body of oxygen, ultimately harming your lungs, although producing a more cerebral effect (due to lack of friggin oxygen and smoke where air should be).
I stopped holding in hits ages ago. breath in, breath out, repeat.
Anyway, UTSE as this has been discussed to death in the past.