I myself have panic disorder and have passed out from a panic attack - HOWEVER if you learn how to breath appropriately with your diaphragm my therapist told me it is impossible to pass out- look into deep breathing/relaxation exercises for more on that...
Panic attacks can last anywhere from literally for a few seconds to a few minutes and maybe longer- if its too much longer though, its more likely just a state of severe anxiety instead of an "attack"
What happens when you come out of the attack?
Well, if you learn the appropriate tools and techniques to deal with a panic attack, you can slowly bring yourself out of it (again with relaxation, breathing, self-talk, etc)and begin to feel calm and eventually back to normal and your body will return to homeostasis