Download counter is reset at the beginning of your billing month. Your billing month starts on the day of the month you signed up an account on DriveHQ. For example, if you signed up an account on 3/9/2006, then your billing date will be the 9th of every month. The download bandwidth doesn't roll over to next month.
If you have used all your download bytes for the month, DriveHQ gives you additional 20% of download bytes for downloading smaller files. You must upgrade to a premium account or upgrade your service to download large files (during the month).