There was a study (https://en.wikipedia.org/wiki/Trinity_study) showing that you could count on a 4% withdrawal rate for retirement income over any 30-year retirement period, including periods spanning the Great Depression and other market lows. Most of the time you can count on more; that was the lowest across the entire study. And with compounding, the difference between "30-year retirement period" and "indefinite retirement period" is not much; withdraw a bit less, or be able to adapt to prevailing conditions, and your retirement portfolio will last indefinitely.