Question: Where will the Dow Jones Industrial Average be in the year 2000?

Don't lay too much money on your answer. A monkey throwing darts probably has as good a chance of hitting the right number as you or I.

Still, it isn't an idle question. Investing is supposed to provide for the long term. If you turn 48 this year, you will hit retirement age at the end of the century. If you are expecting a baby, you will need college tuition money about that time.

Then, too, thinking about the long haul is a useful antidote to the short-term agitation that dominates the financial markets. Money supply up? Buy! Tremors in the Middle East? Sell! Nervous trading makes brokers rich. It has the opposite effect on the rest of us.

Long-term investing used to be simple enough. Triple-A bonds and good-looking equities, together with savings accounts and insurance policies, met the needs of all but the highest rollers. Now all such security blankets are cold comfort, at least to those who remember the wonders of double-digit stagflation. And if economic memory begins at age 10, that includes everyone over 13.

So how to plan? One of the few Wall Street gurus willing to tackle the problem is Peter Bernstein.

This isn't the only way in which Bernstein is unusual. Writer, teacher, scholar, former chairman of the investment firm of Bernstein-Macaulay Inc., he is now a consultant to institutional investors and, in the words of one client, "charges people for his philosophizing," in his twice-a-month newsletter. Nor are his observations conventional. In Institutional Investor magazine not long ago, Bernstein dissected our economic past in an attempt to extract some rules for the future. Starting with 1874, he took overlapping 20-year periods and calculated asset values against price levels, employment trends, and other economic indicators.

A few of Bernstein's conclusions:

* Beware the vision of the Great -- and enduring -- Bull Market. Common stocks, he says, showed "extraordinary performance" during the 1940s and '50s, when their prices rose an average of more than 8% a year. A more typical 20-year stock performance is between 3% and 4% appreciation, uncorrected for inflation.

* If you can't stay away from stocks, then pray for inflation. (Do so quietly, so as not to arouse the wrath of right-thinking neighbors and money managers.) "The stock market," Bernstein notes, "performs better over the long run when inflation is on the high side than when it is on the low side."

* Therefore, rising bond yields go hand in hand with a rising stock market. So don't count on jumping from stocks to bonds and back again.

The next 17 years, says Bernstein, probably won't look much like the last, if only because things somehow seem to change markedly from one 20-year era to another. By this criterion, the outlook is for a period of relatively low inflation, low interest rates, and -- contrary to the experience of the last several months -- not much excitement on the stock pages. Investors looking for something to buy and hold, he suggests, should be looking at bonds rather than stocks.

And the Dow? Judging from the historical record, the average will be only 1,600 in the year 2000. That, he says, is "a good guidepost to keep in mind as we turn gray between here and there."