1- They learned really quickly: the Lutheran and Catholic churches learned about the Eugenic program almost immediately since they operated many hospitals. It's quite obvious when SS enter your hospital and kidnap all chronic sufferers and special needs patients. They send the information out and made a public outcry.

The Christal Night and the dozens of very public lynchings of Jews sent also a clear signal things were getting ugly. Jews began to flee Germany and spread the word.

When Germany invaded the USSR several diplomatic missions were witness of the extreme brutality of the Nazis toward the Jewish conmunity. By then, every politician around the world knew there was a genocide going on.

Recon planes and spies learned of the existence of Extermination Camps in 1942, almost right after the beginning of the Holocaust proper. Resistance members got the word out too.

By 1944, it was in plain sight, massive deportations of people from all over Europe. Allies of Germany like Hungary or Vichy France tried to delay the extermination of their Jewish citizens (debatable/complex).

The Red Army and the USSR also discovered the scope of the Holocaust as they were pushing the Nazis back from Eastern Europe. Mass graves, destroyed cities, and millions of people missing were obvious signs.

The Allies bombed several concentration & extermination camps because weapons were produced there. They knew their existence and location.

For the American public, the full horror was revealed when Patton's army took Dachau. The newsreels were ghastly and didn't hide much.

2- "How did long did it take before the full scope the Holocaust was understood by average citizens?"

The full scope? It was long... It took decades to properly reconstruct what happened. People were aware of the horrors, the striped pyjamas, emaciated bodies and tatooed numbers in 45, but you just have to read old ww2 books to realize they didn't get how bad it was.

After the end of the Cold War, when Russia made the USSR archives public, the last legos "clicked" the scope of the horror began to be really understood by the researchers from the West and the East. The full picture got out only around what? 2000?