Here's a handy way to figure out how long your investment will take to double in value. It is called the Rule of 72.

The rule uses this formula: (Interest Rate %) x (Years to Double) = 72

To find the number of years given a percentage: Years = 72 / (Interest Rate %)
To find the percentage required to double given the years: Rate = 72 / Years

Scenario 1: You have an investment that compounds annually at 7%. Rule of 72 approximates the doubling time as 72 / 7 = 10.3 years.

Scenario 2: You are shopping for an investment that will double in 6 years. Rule of 72 approximates a rate of 72 / 6 = 12% is required.

For more details, visit this page on the Rule of 72 at Wikipedia.