Rule of 72: Take a number very close to 70 and divide that number by the annual rate of return, and that's how long it takes to double.

Rule of 110: Take a number very close to 110, divide that number by the annual rate of return, and that's how long it takes to, ahem, treble. Many people here claim that a 6% real return is average via VTSAX. So $1 today should become $3 in inflation-adjusted buying power in approximately 18 years.