When Is Federal Debt Likely to Reach the Statutory Limit?
from CBO Recent CBO Publications
The Congress has traditionally placed a limit on the total amount of debt that the Department of the Treasury can issue to the public and to other federal agencies. Lawmakers have enacted numerous increases to the debt limit—commonly known as the debt ceiling—some of which have been temporary and many of which have been permanent.

http://www.cbo.gov/publication/43737