How long can a bear market last?

For a gloomy answer to that question, ask anyone in the shrinking band of Wall Street investors who specialize in small-company growth stocks.Fans of "emerging growth" companies get a lot of practice being patient, enduring lengthy dry spells in their quest to discover the budding IBMs, Mercks and Philip Morrises of the future.

But even their fabled stamina has been wearing down of late, given the continued drubbing their favorites have been subjected to in the market decline since midsummer.

By most gauges, the last period of sustained prosperity in their chosen field of expertise ended around the middle of 1983.

"Almost everyone involved in small growth stock investing throughout the 1980s knows how it feels to get up off the canvas," observes analyst L. Keith Mullins in a current Morgan Stanley & Co. appraisal.

How low has the group gone now? Well, at Sept. 30 the price-earnings ratio of the New Horizons Fund was .96 of the Standard & Poor's 500's P-E, equaling lows reached just twice before - in 1977 and again after the '87 crash.

In theory, small growth stocks should carry a "normal" P-E of perhaps 1.5 times that of the overall market, if only because these companies are in position to post greater-than-average profit increases in the future.

By an old rule of thumb, emerging growth stocks are a bargain when their multiple vs. the overall market shrinks much below 1.5, and should be sold as the multiple gets closer to 2.

From Mullins' perspective, "the point is clear. The present valuations for small growth stocks are either a temporary anomaly, or the group has sunk permanently into uncharted valuation waters."