As long as you have a printing press, you (theoretically) never have to default. However, a defact default can occur through hyper-inflation. This will cause all previous debts to be paid off for pennies on the “dollar”. Anyone who is responsible and actually saves money will get screwed.
I think the Market sees this coming, which is why Treasury bonds are getting dumped right now. Of course, the rest of the world depends on US citizens being reliably irresponsible consumers. If that changes, everyone is probably screwed for a generation or two.