Great question. The answer is none. The United States is still paying for debt from the Civil war, and it always will in perpetuity. The United States will always grow it's debt. We always roll it over, It will one day reach 100 trillion. We are not alone in this in the advanced world, and it does not matter.
First of all, the vast majority of US debt is held by the Federal Reserve. SO we owe most of the debt to ourselves. The FED pays the interest on the debt right back to the Treasury.
2nd: Since we have our own currency, the debt cannot ever be defaulted on, since the FED could always monetize debt payment with investor confidence
3rd: Since we are the world's only reserve currency, debt levels don't rattle the currency markets. They do in places like Greece, (who don't have their own currency) or other developing world currencies whose bonds are ALWAYS tied to the dollar
4th: Current USA national level GDP to debt ratio is high, but not higher than it was in 1945. We have done pretty well since then.
Don't worry about the debt.Worry about the those who worry about the debt screwing up policy.