Actually, World War II, at least the part where the U.S. was taking prisoners, lasted less than 4 years. Pearl Harbor was December 7, 1941 and the Japanese surrender was in August, 1945. The German surrender was in May, 1945.

Anyway, the key point here is the specific status of the people at Guantanamo. If they're prisoners of war, they're subject to the Geneva Conventions, which permits detention until the war is over; if they're criminals, they should get tried for their crimes; if they're neither, then we don't have a right to hold them. The habeas proceedings will decide this.