I have left my fair share of new plays and musicals that just weren’t working, and I would probably have left even more if they’d had an intermission (such as Sarah Ruhl’s excruciatingly twee and obvious Eurydice), but I’m still puzzled when people leave opera early.

They know the length going in: It’s generally posted on the opera house website, and most people go to the opera aware that they’ve got at least three, sometimes four and, on rare occasions, five hours to sit through. But toward the end of the evening (and sometimes in the middle of it), you begin to see couples slink out under over of darkness and escape into the night.

I ran into a friend recently just before a performance of Strauss’s Der Rosenkavalier at the Metropolitan Opera in New York. The following day, I asked him if he enjoyed it. He wrote that he did, but that he left after the second act, which surprised me. This man is a seasoned operagoer, and surely must have known he was cheating himself of one of the glories of the opera repertoire by leaving early and missing the sublime trio toward the end of the opera, “Marie Theres'! / Hab' mir's gelobt,” sung by the Marschallin, Octavian and Sophie. But he had a meeting early in the morning.

Then why not just skip the opera altogether? Of course, it has riches in each act, but that final trio is something else.

In a way, it’s like sitting through close to five hours of Die Meistersinger von Nürnberg and deciding it’s not worth your time to make it to “Selig wie die Sonne,” that remarkable quintet in which time seems to stand still as the singers celebrate the good fortune at the composition of a new master song. If you’re already going to devote four and a half hours of your life to this great opera, why not add another ninety minutes and finish what you started?

I realize that people have to get up early for work, and that an opera that begins at 7:30 and ends at midnight is a real commitment. I also realize that many commuters are bound by the tyranny of train schedules, and cannot miss a certain departure otherwise they’d be stuck into the wee hours waiting for the next and cursing their devotion to culture.

I wonder, though, if these folks go into the opera house knowing that they’re going to back out early. Or whether they’ve made a pact with their companion or spouse that they’d go along to the opera, but that they definitely would not sit through the whole thing. Some people are like that about opera. I can understand that. It is an acquired taste (but one that once acquired grows into an insatiable appetite).

Interestingly, few people leave Wagner operas early. Wagnerians are committed to the event, to the length of a Wagner evening (for others they’re like that joke about operas: I sat through three hours of the opera then looked at my watch and saw only 15 minutes had passed). But that’s Wagner for you.

Now, Der Rosenkvalier got me home at a much later time than normal, and I went to bed well past my usual bedtime. I didn’t regret feeling fuzzy the next day, because I still recalled the beauties of the opera, the singing, the experience. After all, one doesn’t attend a performance of Der Rosenkavalier (or Die Meistersinger or Don Carlos or even Don Giovanni) every night. Opera is special, and you make allowances for it, and for the demands that it makes on your otherwise rigid schedule.

It’s not like you can TiVo it and catch the rest later.