Well it depends on what you're doing. If you're just browsing websites, it should last quite awhile. If you do a lot of downloading, it won't go very far at all. For example, a CD tends to be ~60-80MB, while an SD movie tends to be around 700MB. You can see how this could eat up data very quickly.
Again, if you're just browsing, I doubt you'd reach the limit within a month.