When will this bill be worth less than a piece of ordinary (toilet)paper?

Pretty soon
Fears of dollar collapse as Saudis take fright
Saudi Arabia has refused to cut interest rates in lockstep with the US Federal Reserve for the first time, signalling that the oil-rich Gulf kingdom is preparing to break the dollar currency peg in a move that risks setting off a stampede out of the dollar across the Middle East.
China threatens 'nuclear option' of dollar sales
The Chinese government has begun a concerted campaign of economic threats against the United States, hinting that it may liquidate its vast holding of US treasuries if Washington imposes trade sanctions to force a yuan revaluation.