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.
I pay $45 a month for 12Mb\s downstream and 2Mb\s upstream (used to pay $55 for 18Mb\s down 2Mb\s up but never really needed that fast since most downloads never reached full speed due to the server it was coming from so cut back)
That's assuming that you're downloading at 12Mbps (1.5MB/s) for a full hour though, which is pretty unlikely
Actually I get really close to that a lot of times when downloading movies 700MB takes about 10 minutes usually. (though I guess that is about 4 - 4.5GB. per hour not 5 but still close)