Do you want to keep track of how far your usage is from your monthly mobile data limit? there is a feature which lets you keep track of how much of the quota you have left.

1. Head over to Settings > Data Usage.
2. Set your data limit by dragging the orange line to reflect your monthly quota.
3. Set your data usage cycle based on when your "month" starts and ends, and you’re done.

You will be alerted once you hit the limit you have set. Note that the tracked data usage of your phone may vary slightly than your carrier’s tracking.