6.6 years is what a US$1 bill lasts.
The lifespan of Federal Reserve notes varies by denomination and depends on a number of factors, including how the denomination is used by the public. For example, larger denominations such as $100 notes are often used as a store of value, which means they pass between users less frequently than lower-denominations such as $5 notes, which are more often used for transactions.
Denomination / Estimated Lifespan
$1 –> 6.6 years
$5 –> 4.7 years
$10 –> 5.3 years
$20 –> 7.8 years
$50 –> 12.2 years
$100 –> 22.9 years