Einstein on the Beach
It is a single act opera, about five hours long with no intermission. Because of the length and the minimalist (repetitive) nature of the music, audience members are free to enter and leave the opera as they wish. Glass's music tends to cycle round, but does not exactly repeat itself. Admittedly, he has described himself as a composer of "music with repetitive structures". Though his earlier music fits what is normally called "minimalist", he has since evolved stylistically.