so, for instance, to figure out how any megabytes a 56 kb/s (kilobit per second) audio stream consumes in an hour:
56 x 3600 = 201600 kb/hr
201600 / 8192 = 24.609375 megabytes per hour
at that rate, you could do about 208 hours of streaming 56kb/s audio before you would be subject to overage charges.
208 hours/month would be about 6.9 hours/day, if you listened 30 days straight.
FYI, there is good info covering other methods of consuming data at http://www.EVDOinfo.com/5GB