The popular answer is a multiple of spending. The useful answer starts with what you will actually spend.
Start with spending, not income
Retirement replaces spending, not salary. Once payroll taxes, commuting and saving stop, required income often falls.
Withdrawal rates
The familiar rule of thumb assumes a long horizon and a stock-heavy portfolio. Retiring earlier, or holding more bonds, changes the sustainable rate.
Sequence risk
Poor returns in the first years hurt far more than the same returns later. Flexible spending in early retirement matters more than an extra percentage point of return.
