Set a limit! You will be notified when you reach it.

If you have a smartphone runninng on android 4.0+, you can control your internet usage and set alarms and limits to your bundle consume.

Go to Settings >Data usage > Set mobile data limit  > ON

You will be shown a graphic that represents the internet usage for a set period. Click on the option "Set mobile data limit" to set the amount of internet that you want to set as a limit. This way you will be notified when you're close to the line.