I'm not sure about the 550 billion number, but there was a run on the markets that week. On September 19th, the US Treasury put in a guarantee on money market funds [1]. The commercial paper market froze that week as well. That alone put the US economy near complete collapse. Figures such as 3 hours are somewhat arbitrary, but don't dismiss the seriousness of financial crisis that week in September.

[1] http://www.ustreas.gov/press/releases/hp1147.htm