It turns out there's a really simple formula. You just take your spending... So, let's say $25,000. If you multiply that number by 25, roughly, that's how much money you need invested in order to be able to quit working forever. So, if you spend $25,000 a year, you need about $625,000... to be retired forever. If you spend $100,000 a year, you need $2.5 million invested to sustain that living style.