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
It fell in 2010, 2012 and 2013, but it's been
steadily increasing outside those (fiscal) years (and is projected to continue increasing this year and next). It was $16797.5 billion in 2013, having risen from $11512.2 bn in 2003, $6878.7 bn in 1993, etc.
Depending how you measure it (and which economist you listen to!), US public spending is between 30 and 45% of GDP. This isn't an extreme:
there are developed countries that are better, and worse.
I'm not sure I understand why cutting/delaying entitlement benefits doesn't constitute a reduction in public spending?