According to the National Center for Health Statistics, U.S. citizens who make it to 65 can expect, on average, to see 83 years of age. Make it to 75 and odds are you'll be blowing out candles on your 86th birthday cake. Good for us, of course, but if we intend to be jolly good fellows during our dotage, the time to plan is now. Eighteen years in retirement -- and potentially more than 20 -- is a long, long time.

Take the long way home
So how big a nest egg will you need if your life expectancy matches the national averages?

Good question, and it's the exact query that Robert Brokamp -- the Fool's very own retirement guru -- took up in a recent issue of his Rule Your Retirement newsletter. If you're fortunate enough to live a long and fruitful life, the last thing you want to happen is to run out of money.

The 4% solution
As a rough-cut estimate, bear in mind that Robert has done his homework and recommended 4% as a safe draw-down rate for a properly diversified portfolio. If you expect your annual post-retirement expenses to be, say, $50,000, that means you'll need a portfolio in excess of $1 million to make it through 20 years of the leisurely life -- and that's not accounting for the impact of inflation on your purchasing power.

The Fool has loads of sound advice on how to put your money to work, of course. For starters, you could jump aboard index funds, which are no-brainer, low-cost vehicles that give you exposure to growth stocks like Intel (NASDAQ:INTC), Yahoo! (NASDAQ:YHOO), and Research In Motion (NASDAQ:RIMM), as well as more buttoned-down stalwarts like JPMorgan Chase (NYSE:JPM), International Paper (NYSE:IP), Citigroup (NYSE:C), and Kraft (NYSE:KFT). (If you're interested, Robert paints the bigger picture in Rule Your Retirement. You can access all his tips and tactics with an absolutely risk-free spin.)

The Foolish bottom line
Now matter what you do, remember that the prudent course of action is to get going now. After all, as Robert puts it in his newsletter's current issue, "hoping to die sooner is not a very good retirement plan."

Shannon Zimmerman is the lead analyst for the Fool's Champion Funds newsletter service and doesn't own any of the companies mentioned. JPMorgan Chase and Kraft are Motley Fool Income Investor recommendations. The Fool has a strict disclosure policy .