You don't have to make six figures to retire in your 30s or 40s. Plenty of regular people earning regular salaries have shown us how to do it.
But how do these early retirees know when to quit their jobs? How do they know they have enough to fund their expenses for the rest of their lives?
Many of them use a simple formula: the "four percent rule," which says that in most cases you can safely withdraw four percent a year from your retirement savings portfolio.