I'm not on it, but there are a lot of people on here who are. Your doctor is right that it will take at least that long to work and for some people, it takes even longer. It's good that you are on the prednisone while you're starting it. Why the three years? Most people are on it indifinitely (forever, until it quits working, until they can't tolerate it, etc) once they start. I've never heard of someone knowing that they'll only be on it for a set period of time.
Status: Remission since late May 2009!
Symptoms began in November 2008, ~4 weeks after giving birth to my son.
Eased for ~3 weeks in December, possibly b/c of probiotic use?
Returned in January 2009 (with a vengeance), diagnosed with pancolitis on January 30.
Currently taking Asacol (400mg 4 pills 3x daily), Rowasa twice weekly, Culturelle probiotic, and Zoloft (25 mg).