I have my site, with all my songs on it, probably close to 20-25 and I use probably only 1-2gb per month.. not much at all.. not a very heavily visted site either though..
HostingRefuge.com is probably going to hit 3Gb - 4Gb this first month. Maths say it will be at 2.5Gb but I hope to see some good visitors coming and growing daily. Lets hope to see it hit at least 3Gb
I have noticed that most sites don't use past 5 gb unless they are extremely busy or offer downloads/streaming. Of course there are exceptions, such as photo galleries and image hosts but the average site doesn't really use up much bandwidth.
Why do you suppose people buy 100gb bandwidth for $5/month and then wonder why the servers were clogged up or why the service was very poor.