When I'm in the midst of an attack - it can feel like it's continuous for days upon days. However I do have breaks in between where I will feel calmer, just to have another start up shortly after. I think our bodies are so wired from the first attack that it takes time to come down, and the littlest thing can set it off again.
The biggest thing for me is distraction. If you can shift your focus to something other than being in the moment of panic, it can help to ease some of the symptoms.