At only 2% annual inflation the dollar will lose 50% of its purchasing power every 36 years. I believe the loss has been historically greater than that, so our past rate of inflation on average must be higher than 2% annualized long term.
At 3% average annualized inflation the dollar loses 50% of its buying power every 24 years. Since we came off the gold standard, that's probably closer to the truth, though it is probably somewhat worse than that. I can't imagine it being historically much worse (so far) than about 3.5% annualized over the long haul since we abandoned gold. This is mainly because the availability of the likes of cheep Chinese goods has kept a tight lid on (I.E., a mask over) our "real" rate of inflation. That said, if we lived in a "buy American" economy like we had up until about the end of the gold standard, we would realize that inflation in the good old USA was annually well over 3.5% on average.
If inflation was to average 4%, the dollar would buy 50% less goods and services every 18 years. Just imagine it, in only 18 years, everything would cost twice as much as it does today.
At 50% inflation, prices double (the dollar devalues by 50%) every 17 months. Pray that we never experience runaway inflation. Especially with the likes of TARP, QE1, QE2, Twist, and the coming QE3 (some say its secretly already here).
"The greatest shortcoming of the human race is our inability to understand the exponential function." Albert Bartlet (retired Nuclear Physicist)