I don't know how long it took for me, I was on pred for 2 months before I got better for my UC and also my kidneys it took 4 months and it was almost considered a failure for my kidneys (on it for a dual purpose). I find out Thursday how I am doing w/that.
Also, I belive the hydrocortisone enema's are really what jumpstarted my remission. It took about 2 weeks to get some good results.
HTH!