If the rate of price inflation in a currency is X% a year, then in 70/X years, the currency will halve in purchasing power.
If inflation was still calculated the way that it was back in 1980, the official rate of USD inflation would be above 15 percent right now (as per Shadowstats.com)
If this were true then it would take less than 5 years for the dollar's purchasing power to halve.
Shadowstats is funny. They don't actually calculate inflation.