I have one of those with the CellC white modem . it tells you how much data you used and you can check how much you have left on the dashboard ( desk top manager ) http://www.cellc.co.za/internet/page/support/modems/2 i suppose it depends what modem you have
During the promotion the first month was a grace period, means it was not counting to the 12 months. You received extra (free) 2GB valid to the end of the month. Not sure how it works now.