A year is the minimum for us. My daughter is 18m and still going at it. Its only once a day, but we dont have plans to stop any time soon :-) She is sensitive to cow milk, but that didnt have anything to do with our decision.
Both my kids self-weaned about the time they were able to hold a sippie cup on their own and could feed themselves small finger foods, around 15 months. They both just stopped trying to nurse at that time. It was a very natural, effortless process for both of them.