That is a rather open question since people use their phone differently. I'd suggest playing around w/ ATTs data calculator here.
Personally, I typically use just over 200mb. That is 99.9% from email and web browsing. I do not stream video from my iPhone regularly, and when I do its for less than 5min.
I suspect I am "typical" which is why ATT set the new limit at 200 rather than 250 like the iPad. They know people will go over 200 even w/ just irregular use. 250 is a bit easier to stay "within the lines."