How much 3G data does FaceTime video calling use on the iPhone 4?
When using FaceTime on a Wi-Fi connection your data plan is not used. Similarly, no cellular calling minutes are used during the video portion of a FaceTime call.
FaceTime calling can be used over a 3G cellular data connection with a jailbroken iPhone 4 and the My3G package. Using FaceTime over a 3G connection will use your data plan.
Five minutes of 3G FaceTime video calling uses up to 15 MB of data.