How much transfer data do i need on average for my iphone?

I'm getting the 4 iphone. I need to know on average how much people use for their transfer data per month. Like downloading music and videos and stuff. I've heard anywhere from 1 gb to 8 gb and it can get really expensive like 50 dollars per month, so i need to know what i should get.

If you only check Facebook or Twitter than 2GB is plenty.

If you are always on the internet, watching YouTube videos, downloading music or sending emails then 5GB is the bare minimum.