Less than that if there's a major disruption that casts doubt on the legitimacy of more than 5% of the money supply. The classic case was the huge Northern Ban

Less than that if there's a major disruption that casts doubt on the legitimacy of more than 5% of the money supply. The classic case was the huge Northern Bank robbery in Belfast, Northern Ireland in 2004 -- the thieves stole around 7% of all the Northern Bank banknotes in circulation (currency in NI and Scotland is issued by banks that possess a license to print money, not by a nationalized institution like the Federal Reserve). It was cheaper for the bank to reissue the entire currency and cancel the old banknotes than to trouser the loss, so within 4 months the stolen notes were worthless except as toilet paper: http://en.wikipedia.org/wiki/Northern_Bank_robbery http://en.wikipedia.org/wiki/Northern_Bank_robbery Once there's a noticeable proportion of forged coinage or banknotes in circulation public confidence in the currency drops like a stone. At that point, the only option is to recall and replace. The reason this hasn't happened in the USA is because there are so damn many dollar bills in circulation -- around $829Bn as of 2007. The cost of recalling and reissuing that little lot would be distinctly non-trivial ... http://www.ny.frb.org/aboutthefed/fedpoint/fed01.html http://www.ny.frb.org/aboutthefed/fedpoint/fed01.html For the classic currency-catastrophe-caused-by-forgery, you need to read up on the Portuguese banking crisis of 1925, one of the biggest currency frauds in history: http://en.wikipedia.org/wiki/Alves_Reis http://en.wikipedia.org/wiki/Alves_Reis