Fitch says 80% of GDP is the limit

 "Fitch's medium-term fiscal projections imply federal and general government (which includes states and local governments) gross debt stabilising next year and over the remainder of the decade at around 72% and 104% of GDP, respectively. This is below the 80% and 110% thresholds that Fitch previously identified as being inconsistent with the U.S. retaining its 'AAA' status."
http://www.fitchratings.com/creditdesk/press_releases/detail.cfm?pr_id=805079&cm_sp=USecon-_-research-_-Fitch%20Places%20United%20States%20AAA%20on%20Rating%20Watch%20Negative

When will the US debt hit 80% of GDP?  About 2028, based on the last CBO projections.  Debt held by the public is actually supposed to decrease as a percent of GDP until 2019 (when it will be about 68%), after which it will start increasing again.