MANY operas, including great ones, are routinely criticized for being too long. Puccini's ''Tosca,'' a miracle of dramatic streamlining, is conceded to be just right, but ''Madama Butterfly,'' which is not that much longer, is thought to have dead spots. Wagner's ''Siegfried'' is way too long, detractors say, and even many partisans concede as much, not to mention tenors who take on the exhausting title role, the most demanding in opera. Yet ''Gotterdammerung,'' an even longer work, is generally felt to be gripping theater.

The question came up again recently in the critical debate over John Harbison's ''Great Gatsby,'' which just finished its premiere run at the Metropolitan Opera. Those who admired the work were taken with the music itself, which at its best revealed Mr. Harbison's imagination, ingenuity, sense of color, keen ear for pungent harmony and skill at evoking jazz and pop music of the 1920's. The severest critics thought the score faceless and boring. Others found it an honorable effort but lacking in the kind of vitality and musical profile that can create character and propel action. Essentially, they felt, the opera was too draggy, too long.

On one level, whether a musical work is an appropriate length would seem to be a completely subjective judgment, especially in concert music. Schumann, an astute critic, famously referred to the ''heavenly lengths'' of Schubert's great chamber works and piano sonatas. Was he being a touch facetious? Maybe. But what he probably meant was that Schubert's concert works inhabited some cosmic realm where normal time frames are irrelevant. Still, those who acknowledge the ethereal beauties of, say, Schubert's late String Quartet in G yet fault the work for its run-on use of sonata form certainly have a point.

With opera, on the other hand, there are more objective ways to evaluate whether a score rambles on. Operas are music theater works, after all, and opera composers from Monteverdi to William Bolcom have faced the same issues with regard to dramatic pacing and musical content: when to prod the action along, when to let a moment linger, when to animate some dialogue or prolong it lyrically, when to let song alone convey the story, when to diffuse dramatic intensity with a snappy dance or a good gag.