How much data does facetime use on my ipod 4

My ipod touch 4g wgen i go to settings it says. 7.1gb available and 6.6 gb used but mine is a 16gb and when you add it up its only 13.7 and i think it has something to do with facetime cause i call like every day and it lasts for pike one hour and fourty one minutes (that is the longest they usually now last 30 minutes or less ) please help??:)