Want to keep track of where your use of your monthly mobile data limit? If you have ICS or more, it is a feature that allows you to track the amount of quota you have left.
1. Go to Settings> Using Data.
2. Set the data limit by dragging the orange line to reflect your monthly charges.
3. Set the usage of the cycle data according to the time of their "month" begins and ends, and voila.
You will be notified once you hit the limit you set. Note that using tracking data on your phone may vary slightly to track your provider.