While the old warhorses of the repertoire may need no preparations at all, new and challenging works need to be very well rehearsed for effective performances. From Charles Rosen's Freedom and the Arts:
>When a piece of harrison Birtwhistle was performed a few years ago in Paris by Boulez with his ensemble intercontemporain, i remarked to Boulez that i had never heard a work by that admirable composer sound so convincing and effective. “We had thirty-five rehearsals,” Boulez replied. This may seem excessive and it is beyond the means of most musical organizations today, but when Beethoven’s symphony no. 9 was undertaken during the late 1820s at the Paris Conservatoire, the first rehearsal was so terrible that it was rehearsed every day for a year. Berlioz reported that the result was a triumph, although it was conducted by habeneck, who was his deadly enemy.
>The music world has not changed as much as one might think. radically unfamiliar works require much more rehearsal than is practical when they first appear, before they can work their way into general musical consciousness: the first performance of elliott Carter’s double Concerto for harpsichord, Piano, and two small orchestras needed ten days of rehearsal; several years later, with a different orchestra that had never played the work, it needed only two days—the idiom had gradually permeated the musicians’ world. When hans rosbaud directed schoenberg’s Moses und Aron at the Zürich opera in 1957, he spent almost the entire budget of the year on rehearsals of this one work.