Quote:
I don't imagine bills generally last for more than ten.


It appears to actually be less than two [years] - 18 to 22 months for an average 1 dollar bill according to various web sites. Interesting ratios between the denominations:

"The average life of a dollar bill is eighteen months. Five dollar bills last about fifteen months, with twenties remaining in circulation for two years. Ten dollar bills have about the same lifespan as singles do, and the larger denomination bills can last up to eight years."(source)
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/Michael