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.
The US will hopefully be able to "grow" their way out of their debt, as in to have their economy grow enough so that the amount of debt shrinks as a percentage of the US economy while keeping their tax rates (and taxes as a percentage of the US economy the same). This will result in them being able to repay their debt