After figuring out how to use the iPhone 4's FaceTime feature over 3G, the obvious question was how much data does it use? According to a brief test by 9to5Mac, about three megabytes a minute.
9to5Mac conducted a 5 minute FaceTime call "with lots of movement" on a jailbroken iPhone 4, and in total it used 14.7 MB of data. That works out to about 3MB a minute. If you're FaceTiming all the time, that could add up quickly, but if you just call your friend up every once in a while because you feel like you should (feel like you should be using this new fangled video chat function, that is) then you probably don't have to worry about it sending you soaring past your 200MB or 2GB limit. [9to5Mac]