Act without doing;
work without effort.

Think of the small as large
and the few as many.

Confront the difficult
while it is still easy.

Accomplish the great task
by a series of small acts.

Lao-Tze
(Translated by S. Mitchell)
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Author John Bogle points out that the Rule of 72 is more flexible than is commonly thought.

The Rule of 72 is most commonly used to estimate the length of time it would take a beginning amount of principal to double, if it compounds at a given interest rate (typically, one less than say, 20%, as higher numbers tend to work less well). For example, principal at 6% will double in 72 / 6 = 12 years.

What's less well known is that the Rule of 72 is also a way for estimating retirement income. It shows, for a given rate of return, how long you must regularly invest a given sum in order to begin withdrawing that sum without dipping into principal. For example, if you invest 1000 USD per month at a 6% annual rate of return, then in 72 / 6 = 12 years, you could begin withdrawing 1000 USD per month from your portfolio without depleting your principal. In effect, so long as your funds continued to earn a straight 6% annual return, that 1000 USD per month could be your retirement income.1