The longest of commonly performed opera is Die Meistersinger von Nürnberg by (Wilhelm) Richard Wagner (1813–83) of Germany. A normal uncut version as performed by the Sadler's Wells company between 24 August and 19 September 1968 entailed 5 hr 15 min of music.
An opera The Life and Times of Joseph Stalin performed in 7 acts at the Brooklyn Academy of Music, USA on 14–15 Dec 1973 required 13 hr 25 min. Act 7 was deemed by some to be the best.
The Heretics by Gabriel von Wayditch (1888–1969) is orchestrated for 11 pieces and lasts 8½ hours.
Comments below may relate to previous holders of this record.