"To start, we’re offering each Premium individual, as well as plan managers for Family and Duo accounts, 15 hours of listening per month—giving them the ability to enjoy as many titles as they want within that monthly allocation.

[...]

For those super audiophiles who use up their 15 hours before their monthly billing cycles refresh, you can purchase a 10-hour top-up to finish that series. Not sure how many hours you have left? Check it at any time in your in-app settings. "

No cost listed on how much the top-ups are.