How much data does FaceTime video calling use on the iPhone?
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 cellular data connection with an iPhone. Using FaceTime over a cellular data connection (3G / LTE) will use your data plan.
Five minutes of FaceTime video calling uses up to 15 MB of data.