Your data allowance is provided as a monthly allotment that resets on the same day each month for 12 months.
For example, if you join on the 15th of January and receive 2 GB, you will receive 2 GB of data to use until February 14. On February 15, your data will reset back to 2 GB. This monthly cycle continues for the full 12 month period.
If you need more data before your next reset, you can top up at any time through My Account.