investors who bought the Dow at the peak of the market in 1929 had lost roughly 89% of their investment only three years later. They broke even, in real terms, in 1954 -- 25 years later. If you include dividends in the mix, it's a bit better. In that case, the breakeven year was 1945.
Let's look at a more recent example -- the market's fall from its highs in 2000. From peak to trough, the Nasdaq fell 79%. Investors who bought at the peak broke even in ... well, actually, they're not at breakeven yet. The S&P 500 fared better, with investors losing "only" 50%.