Listening to a 128 Kbit/s stream uses approximately 60 MB per hour. Lower bit rate streams (e.g., 64 or 32 Kbit/s) use up less, while larger streams (> 128 Kbit/s) use up more data. Please be sure to tap 'Pause' and stop streaming when you've finished listening.
Small tip: You can deactivate streaming via mobile data in the app settings. The streaming of stations or podcasts will then only work over a WLAN connection.