You should pay attention to what apps your using when you're not on WiFi.
Some apps, like WhatsApp and Snapchat, are pretty harmless because they use a small amount of data, so you'd need send a lot of Snaps and text messages to use up your data plan.
However, some of the most popular apps like Facebook, Spotify, and Instagram can eat up your data plan in a matter of hours. And you can pretty much forget about streaming high-quality video on apps like Netflix and Periscope.
Here's how much data some of the most popular smartphone apps use and how long it would take for each app to use up an average 2GB data plan: