They keep a record of the last 30 days of usage. Whatever the date you got hooked up. So lets say you dont download for 10 consecutive days, then you can download 2.5 gigs per day for the next 20 days. But after your 20 days @ 2.5 gigs/day, you can't download more than 0.25 gig/day for the next 10 days. After 30 days, you cant download/upload more than 1.66 gigs/day if you plan to d/u on a daily basis for the next 30 days...
BBR member itsa
has made a neat little program to help you track your monthly usage (rolling 30 days) to get an idea what numbers Shaw is seeing from your account.
It is available here: »ShawCalc: keep track of your modem usage!