According to Jeff Saut a selling stampede is when the market sees constant selling with only 1-3 session counter-trend rallies. A typical selling stampede lasts 17-25 sessions. Even during the financial crisis this framework seemed to work well, except for a single instance where a selling stampede lasted over 30 days in late 08'.
If we are currently in the midst of a selling stampede today is session 12. It has been a long time since I dusted off this framework as it typically occurs during times of crisis.