Considering that the US is currently trillions of dollars in debt, I would think 5 years, ot 50 years, or 500 years. It would depend on how the government is handling things.
Firstly, the dollar is supported only with the United States of America and the whole world, because each country depends on this currency. And secondly, even such an organization as an international bank will support the dollar, everyone will listen to it because it is this organization that dictates its conditions on the financial economic state of each country.