What to do if i have bad panic attacks, i think i am suffering from depersonalization disorder. how long will this last?

Til you get therapy: Find a therapist who specializes in treating panic disorder, especially helpful is exposure based therapy for panic, where gradually, with the therapist's guidance, you intentionally ; repeatedly bring on various symptoms of panic, interspersed w/ relaxation techniques. By doing this over ; over again, the physical sensations that accompany the panic feeling will no longer trigger a panic attack.