The Trinity Study assumed 30 year retirement invested in S&P 500 with 3% withdrawal rate for a 99% chance of your money surviving you based on historical patterns of inflation and stock market returns. The 3% was adjusted according to the current value of the portfolio. The earlier one retires, the longer the time in retirement and the more assets needed.
If one is interested in moving to a foreign country, your retirement money can go further if you adopt a lower cost lifestyle. But the same can be done in the US by moving to a lower cost state or city.
Those of you interested in the topic might want to join /r/financialindependence on reddit. There are also related concepts of leanFIRE and fatFire based on net worth at retirement.