The long-term harms of Internet advertising on users: Pandora's field experiment [pdf]

"...However, its 21-month time frame and scale-almost 35 million users-make it unusual and notable. The authors write that the effects took almost a year to be realized, and a month-long experiment would have underestimated the long-term effects of the increased ads by a factor of three.

The authors divided the Pandora users in the study into nine groups that were served different numbers of ads; some groups received twice as many ads as others. Consistently, the study showed that as the number of ads increased, users listened less. Serving one additional ad per hour resulted in a 2 percent decrease in average listening time and a 1.9 percent decrease in the number of days a user listened. The results were consistent across age groups, even though middle-aged users listen more hours than younger and older ones."