Most symphony orchestras, even amateur ones, solidify their schedules for the season (Sept through May) by March the previous year. Professional orchestras will have their schedules sewn up a year (or two) beyond that. This includes commissioned works and premiers - so works that have yet to be heard anywhere else.
Operas can take anywhere from three to five years from the point of first work to first major production. There may be workshop performances of the opera, portions of the opera performed to determine what works and what doesn't, but the full opera won't be performed for at least three years after serious work is started on it.
Chamber works can be a bit more flexible, with new works appearing in concerts within six month of conception. Even then, a composer trying to get a new string quartet performed by an established ensemble will find the ensembles schedules booked well into the next year (very similar to that of an orchestra). They may not get to including a new work into their schedule until at earliest, a year away. They may commission a work, but then they're scheduling that work into their season next year, giving the composer time to get the piece done and have it rehearsed - but mostly just to fit it into their schedule.
Universities may produce concerts of newer pieces (with less time between conception and performance) but this is for student compositional works. These concerts are determined well in advance, even if the pieces aren't. So, it doesn't really speak of responding to music trends as it does for staying with established schedules. These concerts are also of student works, not the perhaps more advanced professional composers whose music might be a step up from student works.
What this means is classical music is not responding to changes or current trends in the music world. The pop world is very different. In hip hop or urban music, if an artist comes out with something new, it's possible for other artists to capture some of that essence within weeks. Rock bands move a bit slower, but because the industry is in a large part based on labels promoting artists, if a new artist appears on the scene with a new sound, other artists with perhaps a similar sound will be put forward by other labels within months. Think of the boy band craze in the 1990's. Within 6 months there were five or six groups all releasing albums within the same year. Only a couple of these groups lasted - still, the industry responded to the trends. If a film creates something new that creates a huge interest in the world (the first Matrix film is a great example), it is less than a year other films appear using the techniques made popular by the original. Within two years the Matrix style of special effects and cinematography were in numerous films.
I am not suggesting that classical music start pumping out music at every whim of current trends. Part of what makes classical music classic, is the idea that it is something more than just a fad. And I certainly don't think a symphony written in a few weeks that can be rehearsed in a few days is the kind of music the classical world should be promoting.
However, I do think there should be more opportunities for the orchestras, opera companies and chamber ensembles to respond to new pieces. Part of the positive in the 'pop' model is the Zeitgeist effect - a given piece's popularity is driven by the global embracing of it. If classical music could find a way to harness this effect while remaining respectful of the classical form we would see a marked increase in audience, recording and merchandising sales, and the attendant attention in the mainstream press.
There are a number of ways this might be accomplished. For example, every major symphony orchestra would schedule a "new works" concert, with the idea of accepting new works (composed within the last year) then they could take advantage of recent popularity in new works. If the Berlin Philharmonic were to perform a piece in September that suddenly received rave reviews, then the New York Philharmonic could incorporate the same piece into their "new works" concert in October and so on...
In 1945, "Peter Grimes" was performed at Sadler's Wells, in London and subsequently 23 other opera houses within the next two years. That sort of meteoric rise of a new work is unheard of in modern terms, even though - with the advent of the internet and satellite communications - media is able to spread news around the world in minutes.
There are great new pieces of music being composed today. Yet, it may take a composer 10 years before a piece is performed more than a couple of times.
Why is it we continue to have dozens of concerts every year of Holst's "The Planets" or Bernstein's "Candid" and yet, Corigliano's Symphony No 1 (written in 1991, won a grammy in 1996) is performed less than a half dozen times a year? I love the music of Holst and Bernstein; they are a couple of my favourite composers. But Corigliano is also extremely good and current. He just premiered a new piece at the Cabrillo Festival. How long will it be before it's performed in London, New York, Berlin or Moscow? Too long in my humble opinion.