I can give you a snapshot of my family. My father was USN, and his parents were just here in the States, as was my mother (although she and my dad didn't meet until about five years after the war).

Italy -- forgiven almost immediately. The fact that my family members also viewed them as incompetent probably led to the easy forgiveness.

Japan -- forgiven oddly quickly. My father enlisted the day after Pearl Harbor. I thought he would have held a grudge linger, but he stayed in the Merchant Marine and did a lot of runs to Japan and met a lot of individuals. It was harder to stay mad at that point. My mom also got over their involvement fairly quickly as she (and much of the country) shifted towards the USSR as The Enemy. My grandfather never forgave them; my grandmother accepted them, but it took until the 1960s

Germany -- this is an incredibly mixed bag and a sliding scale. On the one hand there was the divide between East and West Germany, the West being comparatively quick forgiveness (with caveats), while the East wasn't (kinda). My grandfather was a vet of WWI and assumed that all Germans were evil militarists. My grandmother was more worried about Communists than the war. My father only really came to fully forgive the Germans once he took a trip to Germany in the 1980s, although he was well on the way by that point. My mom pretty much forgave the country within a few years after the war.

But then there is the Holocaust.

This horror only really seemed to make its way into the American consciousness in the 1950s, especially after The Diary of Anne Frank was published. This caused a lot of issues for my family. My grandfather saw this as one more example of German barbarism. My grandmother was quietly appalled and it caused her hatred to last somewhat longer. My father was furious when he read the book and it caused one of several splits in his thinking about the Germans. He could simultaneously see West Germany as a bulwark against International Communism, people he had fought against, and perpetrators of something utterly unforgivable. My mom cried when she read the book and mentally divided Germans between Good Germans (most of West Germany and some in East Germany) and Bad Germans (those who had participated in the Holocaust, Nazi leaders, and chunks of East Germany).