And the answer is ... less than a month!
Good job Ma Bell. While T-Mo is the master of gimmicks, ATT takes a measured approach. If a full year of rollover were implemented, we would probably see overage charges go through the roof.
For most of us, we hit the limits once or twice a year. The ATT plan solves that quite simply.
If you regularly need more than your monthly allowance, buy a higher tiered plan.