@FrankoG you receive them during the Collect Income phase. There is some errata I found clarifying:
The United States:
The United States may not declare war on Japan unless Japan first declares war on it or makes an unprovoked declaration of war against the United Kingdom or ANZAC. However, if the United States is still not at war with Japan by the Collect Income phase of its third turn, it may declare war on Japan at that time. Representing a switch from a peacetime to wartime economy, the American player collects an additional 40 IPCs. This wartime economy takes effect during the first U.S. turn in which it is at war with Japan, regardless of the event that triggered the state of war.
https://www.axisandallies.org/wp-content/uploads/2011/12/Axis-Allies-Pacific-1940-Errata-FAQ.pdf