The “Rule of 72” is one of many criteria used to evaluate investments. Simply stated, it gives you the approximate number of years that will be required to double your investment based on the percentage return. All you need to do is divide 72 by the return percentage.
As an example lets say an investment is advertised as offering an average annual return of 12% Using the rule of 72, we know it will take about 6 years to double your money (72/12).
P.S. See this post on how not to get duped by average annual growth rates.