16kb/sec x 3600 sec/hour = 57,600 kb/hour x byte/8 bits = 7200 kB/hour = 7.2 MB/hour = .0072 GB/hour
I don't know what kind of plan you have since they differ from country to country. If you are serious about this, a flat-rate plan usually makes the most sense. I use Verizon here in the U.S. on my Palm Treo - they used to call it an unlimited data plan but now explicitly state the real limit (5GB/month). For audio streaming alone that would mean 694 hours of listening - almost the whole month, 24 hours per day. Not something anyone would do. I think I rarely come even close to using a few hundred MB. Most of my use is email, web, some ATC listening to check new feeds, etc.
We stay with the 16kb/s bit rate to be compatible with the widest range of clients and frailest network connections (dial-up, EDGE, slow DSL, etc.). Higher bit rates do not bring appreciably better quality for plain voice communications, but do limit the max number of connections since we reach a limit almost twice as fast as with 16kb/s (next standard bit rate is 24kb/s).