use a data calculator to estimate the usage
Here's one from AT&T but you can find others with a quick google search ...
http://www.att.com/att/datacalculator/index.html#fbid=6Vkx6NtRwjG
based on this calculator, with a smartphone ...
12 hours of video streaming
6 hours of music streaming
100 posts on social media sites
10 apps/games/sons downloaded
100 web pages visited
is only 1.8GB of Data usage.