I show Supersize Me to my students almost every year, and for most of them, it has an effect for several hours. Usually, by the next day, they are ready to go to McDonald's or another fast food joint. I think one of the problems that prevents the experiement from having more of an effect on the average viewer is that even Morgan Spurlock admits that his experiment was "a bit" extreme. While, yes, there are people who eat fast food every day, Spurlock did not point out anyone else who ate it for every meal, every day. He also includes several exaggerated examples that take away from the logic of his experiment. For example, he compares the cafeteria of a huge middle school and the food that they serve to a very small alternative school for at-risk students in Wisconsin--it's simply an unfair comparison. Obviously, when a cafeteria has three women who have to feed only a handful of students, they can spend more time making healthy meals. The company that promotes that type of healthy food service for schools claims that it could do the same for other normal schools but provides no evidence of being able to do so.
Similarly, when Spurlock orders his meals, look at the size of them--for breakfast, he is often shown eating two different types of breakfast sandwiches, drinking Coke (he never mentions drinking diet sodas or a lot of water), or having some kind of dessert with many of his meals.
One positive that Spurlock's work is credited with is the removal of the "supersize me" option. However, if you go to Burger King and many of the other fast food restaurants, all they have done is taken away the name of the largest meal (Burger King's old king size is now a large). This is almost more detrimental, because people order a large, not knowing that it's the old king, or they might be more inclined to order the large because it doesn't have quite the negative connotation as "king" or "super" does. So, for me, there were just too many flaws and too much bias for Supersize Me to have a much of an effect. I have a difficult time believing that most people do know that they shouldn't eat fast food on a regular basis.