After 21 million bitcoins
If this happens, you are likely talking of the years following 2140.  If bitcoin survives, it is very possible that bitcoin will be worth millions of dollars.  If so, then perhaps single little satoshis will be worth cents or perhaps what we perceive of as dollars.  Especially if bitcoin is used to supercede as the most common method to use currency.  However america would have to lose a lot of value and do something mighty stupid to lose its number one slot as "the preferred method to hold and spend purchasing power"