this is what Mike (mtindor)had to say when asked in another post
Assuming you'll be doing like everyone else here who provides a feed to RR, and assuming you are running 16 kbit bitrate, then:
16 kbit/sec * 60 (seconds in a minute) * 60 (minutes in an hour) * 24 (hours in a day) * 30 (average days in a month).
I provide a feed using a 16 kbit bit rate. That equates to an average daily figure of 1382400 kilobits, 172800 kilobytes, or about 168 MB a day... which is about 5 GB a month.
8 kbit stream = ~2.5 GB a month
16 kbit stream = ~5 GB a month
NOTE: It doesn't matter how busy or how idle your feed is. As long as your computer is pushing a feed to RR, it'll be using bandwidth. And, as far as bandwidth usage that RR distributes to clients who are listening, that is totally dependent upon the number of listeners at the time -- and is irrelevant from your standpoint.