The public view of him had been crumbling under the facade once people started realizing that the war was not winnable anymore or at least not as successful like the Nazis claimed, leading to further suspicions in regards to its possible overcome. Some people thought so after Stalingrad, others after Kursk, others after D-Day or the first time the USSR reached German territory in East Prussia. Eventually, as Soviet forces stood in Berlin and the Americans in Cologne, only the most delusional Germans still believed in victory.
Even though the belief in the Endsieg (eventual victory/final victory) was heavily propagated by the Nazis, people still noticed and asked themselves why there had to be the formation of such desperate units like the Volkssturm.
That said, defeat still came as a shock to a great number of people, and suicides were prevelant throughout society, especially by those who had supported Hitler.
Stories of Hitler's, Goebbels' and Himmler's suicide quickly turned those who were still torn about the entire thing sour towards their former leadership.
Then, the Holocaust and how much the allies told the Germans, both captured veterans and civilians at home, about what they had been supporting all these years, put a final nail in the coffin.
So, to sum it up: Defeat -> Leadership Suicides -> Holocaust.
However, it should be noted that there were some that held steadfast in their Nazi beliefs. Heinz Guderian, father of the armored Blitzkrieg for example, was pretty active in secret Neo Nazi circles attempting to overthrow the newly reborn (West) German Republic.
Another example was the SRP, the Sozialistische Reichspartei (Socialist Party of the Reich), which openly demanded, as part of its democratic platform, a solution to the Jewish question, even though they mildly criticised the Holocaust. The SRP was supported by many former Nazis.
But you asked how quickly Hitler was viewed negatively. Well, there are differences in opinion there. Zentner writes that the German people turned on him rather quickly, Piekalkiewicz notes that the German civilians, many now without adult males, could not be bothered to think about politics in the first few months and years after the war. They only started truly believing in their government again after the early 1950s brought both German states (but the western one more so than the eastern one) some sort of prosperity and perspective for the future.
And by that point, it had been clear that democratic movements had rebuilt the country that nazistic movements had destroyed; and hence an anti-nazistic pro-democratic consensus, supported by extensive education programs in regards to Nazi war crimes, was formed in all of Germany.