The 4% Rule is a general rule-of-thumb that says in retirement, you can withdraw 4% of your investments annually without risking it running out. It's also called the Safe Withdrawal Rate.
You can invert it to calculate how much you need to save for financial independence: 25x times your annual spending.
The rule came from the Trinity Study, where the authors backtested historical market data for various portfolios from 1925 to 1995.