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the answer is that there is no set frequency in which crashes occur. It really depends on market conditions, which in turn are driven by sentiment and long-term global economics. If anything, history shows us that crashes come in streaks with long gaps in between the streaks.
Looking at the recent data there are two interesting points to note. The first is that we've had two major crashes in quick succession (2000, 2008) during a 15 year consolidation phase. The second is that market volatility has been very low since 2009 which points to us moving into a much more trending phase of the market once again. If that assessment is correct, then it could mean there won’t be another crash along for quite a while.
As an aside, it’s the availability heuristic and recency bias that leads people to think market crashes must be a regular occurrence, even though the data doesn't appear to support that premise. In fact it’s highly likely people in the 1960s thought that market crashes only happened once in a lifetime, since there hadn't been one for over 30 years.