The magic number 8 is just an arbitrary number. Most of the charts usually takes 8 to 10 days to correct. This number can be vary depends on the condition of the market and the particular stock. Currently there is no statistic done to determine the average number of downward trends for each stocks. However, after a significant number of downward trend days it is normal that the stock back to upward trend and it works like a cycle most of the time. It is market cycle plus cycle of that particular stock.