If you activate the service on the 15 or 30 day Unlimited Data plan you will get 7GB of high speed data for that service period.
If you activate the service on an ongoing basis your service periods will be calendar monthly based and you will get 7GB of high speed data each month; i.e. the allowance is reset on the 1st of each month.
Data is unlimited, but should you use more than 7GB, your data speed will be reduced to a maximum of 128 kilobits per second (2G equivalent) for the remainder of that particular service period.