One source (Aaron Green) states, "The shortest opera is Darius Milhaud’s The Deliverance of Theseus. It lasts only 7 minutes, and that’s without intermission".

Peter Reynolds' "Sands of Time", is listed in the Guinness Book of Records as the world's shortest opera at three minutes and 34 seconds long.
"The shortest published opera is The Sands of Time by Simon Rees and Peter Reynolds, which lasted 4 min. 9 sec. When first performed by Rhian Owen and Dominic Burns at The Hayes in Cardiff, Wales, in March 1993. A 3-minute, 34-second version was performed under the direction of Peter Reynolds in London, England, in September 1993.

My answer?
Peter Reynolds' "Sands of Time", either way, he's got the shortest!

:)