For instance if you have $100,000 and you want to know when it will be $200,000 then assuming you made a 7.2% return it'll double in value in 10 years. Conversely if you made 10% compounded over 7.2 years, then you'll double your money again.
Also, the rule says that to find the number of years required to double your money at a given interest rate, you just divide the interest rate into 72. For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years. Eezy Peezy!