How much data will i use on an iPhone?

I'm going to be getting an iPhone soon but i was wondering how much data i will need on my plan. Most of the time i will be on Wi-Fi like at my house and at work and probably school. However when i'm out and about i want to be able to use some data on my phone. I know its pretty hard to know how much data i will use but lets just say i use Facebook or instagram a few times and maybe Pandora or some sort of gps app during the time i'm using up the data, how much do you think i would generally use a month? Sorry this is pretty vague i'm just trying to get a better idea how much data i will need to get on my plan!

2G for every month daily play & read & watch on your iPhone

As you have other sources to use, I think just one gbite of data is enough according to many years of just experience!