The payout periods they looked at was 15 and 30 years. They determined that at a 3% to 4% withdrawal rate per year could make your money last for 30 years. Of course, 3% is better than 4%, but over the course of those many years through the Great Depression, stagnant 60s, inflation periods of the 70s, and booming 80s, their research showed that it is safe to withdraw 4% of your retirement every year.