over a few days a lot over a month not a lot.
100mb of data doesn't take a long time to burn through but again it depends what your doing.
if you preview songs on the itunes store it will be around 1mb per song on average.
browsing the cover art could be 10k of data.
mail is an interesting one i usually get all the signatures that people love to do as images which burns some data that way be it again only little amounts but it all adds up.
all depends on how much email you get every day, my suggestion would be install istat menus and checkout the network tab.
it will tell you how much data you've done.
close everything down except IM and your mail and do some fetching over the weekend this will give you an idea of how much traffic you go through.