A movie is considered a flop based on the number of people who go to see it in the theaters. It usually can be determined based during the first weekend that this movie is in the theaters. Besides the number of people who come to see that movie, it also depends on the money that movie made. If a movie doesn’t get a lot of people to see the movie and therefore doesn’t make any money, it is considered to be a flop.
Most of Tom Cruise’s movies are good and make money. They are not flops, but there are some that do not make as much money as expected. The movie Rock of Ages is a flop because it cost more money to make it than what it made in the theaters.