How long can they prop up the dollar before it all collapses? No country has ever been able to do it. They can prolong the inevitable, but only for so long. The Canadian dollar is almost at par with the US$ now, which underscores how the US$ has declined. Just saw a sign in a gas station yesterday "US $ at par". What a change in 5 years, I remember getting an extra 50% on the exchange from US$ to C$.
The world economy seems to be getting stronger now and is not as reliant on the US as before. All the US companies that have some operations outside the US won't be hurt as much, they'll be pulling in rising foreign currencies. Many top economists in the US have predicted huge problems for the US economy, due to all the government debt and foreign countries holding huge US$ accounts, but the government keeps finding ways to patch it up temporarily. Something's got to give and it won't be pretty. But you can never count out US know-how and determination, so who really knows what will happen.
As a worst case, you'll start seeing more regular people investing in gold and switching their US$ to stronger foreign currencies. If I was an American I'd be seriously looking at all my investments that are in US$, and finding ways to convert them to something safer.