Pandora has capped free listening to 40 hours per month, apparently slightly more than the average listener uses. Via Pandora’s blog.

Pandora's per-track royalty rates have increased more than 25% over the last 3 years, including 9% in 2013 alone and are scheduled to increase an additional 16% over the next two years. After a close look at our overall listening, a 40-hour-per-month mobile listening limit allows us to manage these escalating costs with minimal listener disruption.

Straight up politics in the effort to lower royalty rates.

As the most rights friendly tech guy I know, rights owners still don't seem to get the full implications of not letting these services get to scale. Five, ten or even twenty-five million active users isn't scale on the internet.