I think it may be different for each individual. I know I have a personal favorite that always helps me...
...but I have heard/read of other techniques that people swear by for themselves.
I usually practice mine for just a few rounds until my symptoms subside.
However, regular pratice, a couple of times a day for 10 minutes or so is optimal for lowering overall stress levels.
Let me know if you are interested in my favorite breathing exercise, and I'll post it for you.
"Courage is not the abscence of fear, it is feeling afraid and doing it anyway!"
"Worrying does not empty tomorrow of its troubles...it empties today of its strength."
Corrie Ten Boom
Panic Syndrome recovery due to CBT