Digital Music News has posted an interesting story looking at how long it would take for a stream to equal the pay of a purchased download. Assuming a song would be played once a day, every day the site has come up with some estimates using royalty percentages based off artist testimonials.
According to the report, on the high end a consumer would have to listen to the same song once a day for 700 days straight on Pandora for an artist to earn the equivalent of one paid download. While the methodology of the numbers may come into question it's apparent why artists and their managers would much rather fans buy a download versus stream a song.