At its Dresden premiere in 1842, "Rienzi" lasted about six hours. Much of this music has been lost in the past 170 years.
The most complete performance in modern times was conducted by Edward Downes on the BBC in 1976. It timed in at 4 hours, 40 minutes.
First runner-up is Heinrich Hollreiser/Dresden Staatskapelle on EMI: 3 hours, 38 minutes.
The Downes/BBC "Rienzi" was issued on the Italian label Ponto in 2005: Yeah! Unfortunately they forgot to tell the BBC. Boo! It has been withdrawn.
The Ponto version of the Downes/BBC Rienzi is available as a download (6 euros) on opera-club.net Recommended as a stopgap until we get an official CD release authorised by the BBC. And if the BBC does decide to release it, I hope they don't forget the libretto and translation (now that I'm at it, I wouldn't sneeze at a new uncut recording conducted by Barenboim, Janowski or Thielemann).
"Complete" recordings of Rienzi, in ascending order of completeness: [hint: for ease of navigation, read the review though to the end, then come back and click on the links.]
1 hour , 59 minutes = Josef Krips, Vienna State Opera (Set Svanholm, tenor), 1960 (mono) : Wagner: Rienzi
2 hours, 17 minutes = Lovro von Matacic, Stuttgart Radio (Wolfgang Windgassen), 1957 (mono) : Wagner: Rienzi
3 hours, 38 minutes = Heinrich Hollreiser, Dresden Staatskapelle (Rene Kollo), 1976 : see below *
4 hours, 40 minutes = Edward Downes, BBC (John Mitchinson), 1976 : download on opera-club.net
The 2 hours, 41 minutes separating the longest from the shortest performance is unprecedented, and more than a little ridiculous.
Adolf Hitler may have inadvertently killed Rienzi when word got out that it was his favorite opera. This seems unfair: After the war, his valet reported that Hitler's favorite movie was "Snow White and the Seven Dwarfs" yet Snow White seems to have survived unscathed.
The original score was lost in a World War II bombing raid. A roughly two-and-a-half hour version was thought to be all that survived - until the 1970s when musicologists pieced together an additional two hours from surviving fragments. This was the basis for Edward Downes' pioneering 4 hour, 40 minute BBC broadcast in 1976.
Unfortunately, subsequent performances did not take the hint. The Hollreiser/EMI studio recording (also 1976) is the longest version ever issued commercially. Even so, it is fully one hour shorter than Downes. Since then, the opera has gotten progressively shorter. I sometime think I am the only person who cares about this. I wouldn't mind it if Opera Houses wanted to present abridged performances, if only ONE uncut Rienzi was available on CD.
* For the time being, your best bet is Hollreiser/EMI, available in three different editions:
Rienzi - includes printed libretto and translation