If past is prologue, the Dow Jones Industrial Average will hit 25,000 in about five years. Yes, you read that right. Of course, if history were any kind of reliable guide, past performance would indeed be indicative of future returns — and investing would be easy.

Still, average historical blue-chip returns after a plunge of at least 30% like the one the Dow suffered in 2009 are rather stunning, according to data from Chart of the Day. Going back to 1900, the Dow has logged 13 major rallies after dropping 30% or more. The average rally has lasted roughly 2,000 trading days, or about eight years, and those rallies have generated an average gain of nearly 300%.

See the chart for yourself, courtesy of Chart of the Day:

Source: Chart of the Day

The current Dow rally is running a bit behind its historical average in both return and duration — but then, it still has time to catch up. The Dow has gained 97% from its March 2009 low. If the blue-chip index follows its 111-year average return over the average eight-year rally span, the Dow will pass 25,000 sometime around the second quarter of 2017.

Yes, it sounds too good to be true. On the other hand, the outlook for stocks hasn’t been this bright in years. Even Nouriel Roubini, the economist known as “Dr. Doom,” is bullish. Gluskin Sheff’s presciently pessimistic David Rosenberg is bullish on certain select areas of the market, including certain equity sectors.

Of course, there are plenty of reasons why this rally could fall short of historical returns. The most recent rally certainly did. It ran just five years — from 2002 to 2007 — and petered out after a 100% gain. The credit crisis that ended that last rally so abruptly is still very much with us. Just have a look at the headlines out of the euro zone.

Perhaps more important is that even if we do get to Dow 25,000 in five years or six years, far too many investors will almost certainly rob themselves of returns, because stocks never move in a straight line. Human behavior being what it is, how many investors will bail out at the first sign of trouble?

The Big Kahuna of all Dow rallies illustrates the point. After bottoming in 1942, the Dow rallied for more than 7,000 trading days, resulting in a gain of about 1,000%, according to Chart of the Day. But take a look at a chart of the Dow over that period and you’ll see that investors had to remain almost impossibly frosty amid some truly horrific selloffs to achieve that return.

Click to Enlarge The Dow’s remarkable rally from 1942 to 1973 encompassed many waves of steep, sickening drops, as this chart from S&P Capital IQ reveals. The 1960s alone were stomach-churning times. In two of the most glaring examples, the Dow plunged from about 700 to 525 in early 1962, and from 1,000 to 800 in 1965-66.

More recent wealth-destroying volatility has made investors even more likely to hurt themselves, notes John Hussman, the well-regarded manager of Hussman Funds.

“The S&P 500 has lost more than half of its value on two separate occasions since 2000, and the value of avoiding major losses in a decline generally offsets missed gains very quickly,” Hussman writes in a new note to clients.

For example, a 20% loss wipes out a 25% gain, a 30% loss wipes out a 43% gain, a 40% loss wipes out a 67% gain, and a 50% loss wipes out a 100% gain, Hussman notes.

“My argument is certainly not that stocks will decline immediately, nor that they cannot advance further from present levels,” Hussman writes. “Rather, the point is that even if such an advance emerges, the likelihood of those gains being retained by investors over the course of the full market cycle is exceedingly small.”

So even if past is prologue and the Dow rally continues on historical course, investors will need to hold fast — and keep emotions in check — if they are to reap the market’s rewards. Don’t be your own worst enemy. As they old saying goes, manage the downside, and the upside will take care of itself.