Isaac Asimov
Easily, the longest opera would be Wagner's The Ring Cycle which was written in four parts: Das Rheingold, Die Walkure, Siegfried, and Götterdamerung. It lasts around 18 hours (with intermissions) and is normally performed in four shorter performances. By contrast, the longest single performance would be Wagner's Die Meistersinger von Nürnberg which is around five and a half hours long. But then..."long" is a relative term and much depends on whether or not you're enjoying the experience. If I'm not mistaken, The Ring technically is considered to be a tetraolog(or triology, with prologue): a grouping of 4 different operas, not a single opera: it therefore would not qualify as a correct response. Also, an uncut version, usually last 15-16 hours, depending on the conductor(s) tempi: but not 18. Loge!
Moulin Rouge
A diggleluck is a gnome. The Divide Triology was made by Elizabeth Kay and is a big hit.
Triology
Cornelia Funke .
Robert Zemeckis
Unfortunately, there isn't information on the 6th Professor Layton yet. Although, there definitely will be a 6th one, seeing as a triology finished (3 games) and there is another triology planning to come out (3 more games). So if there isn't another triology, the 6th game will be the last of the Professor Layton series. Keep watch for news on the beginning of the 2nd triology of the Professor Layton series, the 4th game.
Jeff Watts
Trilogy derives from the Greek trilogia, three+story. Triology is someones error. Current dictionaries are of no use - they consider anything ever written or spoken to be correct .
The Mass Effect Triology.
The Bounty Triology
Isaac Asimov
plumber