Immediately. I would even say that they were present during the final year of European war as well, and before. Rumours would spread among villages and towns around the various labor and death camps, and many people did not believe them; including members of my family, who are from a small village north of Vienna near the Czech border. If you were not part of the underground resistance movements, I would assume you most likely would not have been aware of the extent of the bloodshed, or your own psychological barriers would inhibit you from accepting it. That being said, much was done at the end of the war in terms of propaganda in order to cover up the true manner of the Final Solution. The policy surrounding the FS also facilitated denial after the war as there are little to no official documentation of any genocide being ordered, as the vast majority of orders were made verbally. Denial materials began to circulate after the war, such as Friedrich Meinecke's The German Catastrophe (1950). Also, Paul Rassinier published Le Passage de la Ligne (1948) and The Holocaust Story and the Lie of Ulysses (1950).

My sources are mostly personal communications with family, and a jumble of content I have learned in my History bachelors degree.