Does it show the Kbps rate (an indirect indicator of quality)?
If it does, you can calculate the MB per hour of listening.
If not, take note of the 3G data counter before and after 10 hours of XM listening and divide by 10.
I have the infrastructure to measure MB/h, so if you PM me your XM credentials I can test it for you.
... Syche!
But I really can measure Kbps on my 3G link.