Hello Carly,
I believe that I had read that typically they peak in 10 minutes or so but the tapering off could last of several hours.
I know there are many factors and everyone is different, but in many cases it is the body that is tense in some way that triggers the mind that it's in danger and in return the brain continues to produce adrenaline.
For some, attacks maybe as short as 10 or 15 minutes and for others overall, much longer. Some even have multiple attacks over several hours. The key would be to relax in some way...if you go to the anxiety main page, third post down...you will find several techniques that may help shorten the time of the anxiety....good luck and best wishes....WM