You can estimate your usage if you know roughly what you do with your connection, for example:
Downloading a piece of music (a single 4-minute track, known as an MP3 file) could be expected to use about 4 MB.
A 5 minute single streamed broadband music or movie video is around 13MB.
Watching a streamed broadband video for one hour is around 150MB/hour
Listening to one hour of online radio is around 18MB/hour.
Playing an online game like Counterstrike is around 10-12MB/hour
Email is generally very small, 300 emails use around 2MB, but if you add spam and emails with virus attachments this can rise to say 6MB for 300 emails.
Receiving or sending still pictures might be expected to use anywhere between 0.05 and 0.25 MB per picture
Receiving or sending relatively short word processing documents and reading largely text-based Internet pages (browsing) would commonly use approximately 0.02 to 0.05 MB per activity.
Web browsing averages out at around 10MB/hour (Megabytes per hour