Really they have no way of knowing. I would always go with a rule of thumb to keep it under 5gb. Not sure what you were going to use it for but I use it several times a month (school, DL, etc) and the most data I used was 1.5gb. Just track it thru the month using #3282 send from your phone. There are 2 to look at PDANET and Easytether.