The folks over at 9to5mac have found that FaceTime uses just under 3MB of data per minute. To be able to use FaceTime over 3G you need to jailbreak your iPhone 4 and then download the My3G app from Cydia. Then you can ‘trick’ the iOS into thinking it’s on WiFi when it’s actually using the 3G cellular network. Obviously this has it’s upsides such as using FaceTime where ever you like or watching high quality YouTube videos on the go. Bear in mind if your using FaceTime on 3G then take into your bandwidth cap into consideration. On O2 in the UK currently most iPhone 4 users have a 500MB+ cap. So if your on the 500MB plan then you can use FaceTime on 3G for a total of around 2 1/2 hours a month, that’s just on FaceTime. So make sure if you are using FaceTime on 3G that you don’t use it too much.
FaceTime uses 3MB of Data/Minute