To estimate the data transfer rate of your website, you need to estimate the size of your web pages and estimate the amount of page visits.
My blog site is about 6000+ visits per month, the bandwidth used is about 400+MB. (my site has very small amount graphic)
If your site has a lot big size files(graphic, music...) then the bandwidth requirement is bigger.
Normally, 2GB per month is enough for a personal website.
Hope this help you to estimate how much u need.