It fluctuates wildly based on the whims of who is in charge, but the last alterations of the rules indicates 90 days of US imports (doesn't specify usage).

"International obligations

As a member of the International Energy Agency (IEA), the United States must stock an amount of petroleum equivalent to at least 90 days of U.S. imports. The SPR contained an equivalent to 141 days of imports as of September 2016. The United States is also obligated to contribute 43.9% of petroleum in any IEA-coordinated release."

https://en.wikipedia.org/wiki/Strategic_Petroleum_Reserve_(U...