iTunes is finally letting its song previews play just a little bit longer.

Apple has, as promised, begun extending the length of song samples from 30 seconds to 90 seconds at the iTunes Store in the U.S. Many samples of the site's top-selling singles, such as The Black Eyed Peas' "The Time (Dirty Bit)" and Coldplay's "Christmas Lights," were available tonight in the longer format.

The length of song samples has been found to be key to a consumer's decision to purchase music online. Researchers at Robert Morris University reported last year that consumers were more likely to buy songs if allowed to sample the music for about 60 seconds and if provided access to a "high-quality" version of the music.

Apple confirmed last month that it planned to extend the length of samples from 30 seconds to 90 seconds for songs that are at least two-and-a-half minutes in length. Shorter songs would continue to be offered in the 30-second sample. In a letter sent to music labels announcing the move, Apple said, "We believe that giving potential customers more time to listen to your music will lead to more purchases."

As first reported by my colleague Greg Sandoval in August, Apple was expected to announce the increase in sample lengths during the press event in San Francisco on September 1. However, that announcement was called off after National Music Publishers Association informed Apple that it had not negotiated for the proper licenses with music publishers and didn't have the right to offer longer samples.