During the 2010 Flash Crash, about how long did the most violent selling last?

During the 2010 Flash Crash, the most violent selling lasted about 36 minutes.

On May 6, 2010, US stock markets experienced an extremely rapid decline and partial recovery. The Dow Jones Industrial Average fell more than 1,000 points, nearly 9 percent, within minutes before recovering much of the loss. The event affected equities, futures, options, and exchange-traded funds.

A joint investigation by the US Securities and Exchange Commission and the Commodity Futures Trading Commission found that a large automated sell order, combined with stressed market conditions and high-frequency trading activity, helped intensify the plunge. Later analysis identified a trader, Navinder Singh Sarao, whose spoofing activity contributed to the conditions, although the event had multiple interacting causes.

The phrase “36 minutes” describes the core period of the sharpest market disruption, not the entire trading day. Some individual securities experienced much stranger price movements than the major indexes.