According to “American Veteran Aid” here are the eligibility requirements and eligible periods.
World War II: December 7, 1941 – December 31, 1946, inclusive. If the veteran was in service on December 31, 1946, continuous service before July 26, 1947, is considered World War II service.
Korean conflict: June 27, 1950 – January 31, 1955, inclusive.
Vietnam era: The period beginning on February 28, 1961, and ending on May 7, 1975, inclusive, in the case of a veteran who served in the Republic of Vietnam during that period. The period beginning on August 5, 1964, and ending on May 7, 1975, inclusive, in all other cases. (Note: From February 28, 1961 to Aug 4, 1964, vet must have boots on the ground.)
Persian Gulf War: August 2, 1990, through date to be prescribed by Presidential proclamation or law.
Surviving Spouse:
$1,228 Monthly | $14,742 Yearly
Single Veteran:
$1,911 Monthly | $22,939 Yearly
Married Veteran:
$2,266 Monthly | $27,195 Yearly
Two Veterans Married:
$3,032 Monthly | $36,387 Yearly