In theory, there is no limit to how far the stock market can decline. The stock market crash of 1929 ended up with an almost 90 percent loss of market value when that bear market was finished. Although investors expect the market to increase over time, values can and do drop.In theory, there is no limit to how far the stock market can decline. The stock market crash of 1929 ended up with an almost 90 percent loss of market value when that bear market was finished. Although investors expect the market to increase over time, values can and do drop.