It'll never run since they have the power to demand that it be kept sufficiently full.
How, exactly, are we going to demand sovereignties like China or the EU give us trillions of dollars?
As a serious country you can just keep servicing the bonds with bonds. If the government is... well.... it starts to look more like one of those poker websites where you cant cash out?
The world is full of similar but poorly organized ..., trump is doing one hell of a job uniting them. It seems a lot like that outside, universal threat to make us recognize this common bond.
I combine some "hallucinated" numbers into a table here