It just depends, some on your body, and some on the doctor. My first flare lasted 10 months; my doctore wasn't a GI and tried sulfasalazine without prednisone. If you have a particularly aggressive flare, it can take months to break it, with multiple drugs, etc. until you get the right treatment. Recently I had a flare that lasted a year, which was partly the doctor's fault, because he refused to give me Remicade and tried to have me stick it out with the pred and Pentasa.
Some doctors attack Crohn's aggressively with biologicals and immunosuppressants, and some prefer to save the big gun drugs for later, giving you the first line meds first and see how you respond to these.
Sometimes you can have an underlying infection or condition that aggrevates the flare; once these are treated the flare will stop. I know it's frustrating, but alot is trial and error, and also learning how your body and disease responds to treatment.
Hope this helps; don't let it go on too long, though, as this can cause problems for you down the road, in my experience.