How long did the phony war last for in World War II?

The "phony war" or "sitzkrieg" refers to the period of time between September 3, 1939 and April 9, 1940 when there was little or no air or ground combat on Germany's western front. It began with the German invasion of Poland to start WW2. It ended when Germany invaded Denmark & Norway on April 9, 1940. Some say it ended when Germany invaded France, Belgium, Netherlands, and Luxemburg on May 10, 1940.