Never, the only way the government could pay it off in a traditional sense is if they somehow tax [current level of debt] out of the us citizens. That I think will not happen, so they will either default or hyper inflate their debt away.
Or they could reduce public spending. Britain in the early 1980s did this, and the Public Sector Borrowing Requirement became (temporarily) the Public Sector Debt Repayment.
Public spending in the US is already very low, the lowest it has been in decades. I do not think we can cut pubic spending very much more. It would be better (and necessary) to cut/delay entitlement benefits so that the taxes that pay for entitlements are more closely aligned with the amount of benefits that are paid out, over the long term
Deficit and debt piling up every year said otherwise.