I have UC and MS as well. My MS has not been active.
I was diagnosed with severe pancolitis in June 2008. I have basically been flaring (thankfully not severely) since January 2009. I have been on prednisone since then but haven't gone above 10 mg in months. I am having trouble getting off of it, though, but am not ready to try 6MP mainly because I am severely anemic and trying to get my iron levels back up.
As far as I know, 6MP can take about 4 months to kick in, but definitely talk to your doctor if you have doubts.
I hope you feel better soon. You are not alone, and this forum is the best.