If you spend $k/year and make $k/year after taxes, you're saving % of your income ($k/year).
Put another way: for every year you work, you're earning years time off.
If you can put your money in an asset that earns % interest, you need x your expenses ($k) to live indefinitely on interest. At the rate you're saving and earning interest, it will take about years to save $k.
Alternatively, if you start a business that makes $ profit per customer per month, you'll only need customers to cover your living expenses!