my 2.5 year old gave hers up a month ago....she only had it for bed, until one day she rubbed coleslaw into the arm of the sofa. We fell out and I told her she needed to do something to make it up to me, I asked if I could have her dummy and chuck it in the bin. She agreed, after that she only asked for it once or twice and was reminded that I wasn't amused about the coleslaw - can't quite believe it happened like that, I know it sounds a bit mean but we had talked about her giving up quite a few times, and I think her teeth were beginning to look a little crooked.
Every child will be different though, perhaps offer a reward if she can go a week without it. Try not to make a big deal out of it but once its gone don't go back!