For some people Remicade just one day stops working and others have remained on the drug for years with great success (there are a few on the boards here). I have been on Remicade since Jan. 2007 with great success for over a year and now I am flaring on Remi. So my doc is upping my dose from 5 mg to 10 mg. The docs can play with your dosing- every four or six weeks instead of every eight weeks. Thus- if things start to go bad it may not necessarily be the end of the road and Remicade can still work for you. Be positive and have some faith (at least that's what I try to do).
The next treatment options are Humira or surgery. Humira is not yet FDA approved for treatment of UC, is crazy expensive, and may require a struggle with insurance to get them to pay for it. Humira is a tnf blocked like Remi, but is administered by a shot (vs. a Remi transfusion) and is dosed every two weeks (although some require weekly shots per my GI), and is human protein based rather than mouse protein based like Remi (which is why Remi just does not work for some people). There are other meds that are in clinical trials in the U.S. and abroad for UC- but for the most part their true effectiveness as a treatment for UC is unknown.
If you are doing well on Remicade now my advice is to enjoy it and be positive that it will continue to be a good option for you. It is always good to be informed about "what's next", but I wouldn't put my focus there. Good luck